I'm so sorry this happened to you! I had the same thing happen to me recently, same exact pop up when I applied for the Platinum Card in March 2021. I think my Amex Gold card I got in November 2020 with the 60K MR points is the culprit. It doesn't help either that I opened a lot of cards from December 2019 to January 2021. 13 to be exact. 2 were Amex business cards, so on paper I'm 11/24 but in Amex eyes they can clearly see I opened 13 cards. All 13 cards are keepers for me though, I have no plans to close any of them.
I've gotten approved for every Amex card I've applied for until I applied for the Platinum. I was 4/4 (my 1st Amex is an authorized user card). I was denied in February for my Amex Account being too new. That was gonna be my 6th Amex card since December 2019 if you include the card I'm an AU on. I used the Amex pre-qualify tool in March and saw I was pre-qualified for the Platinum. I applied and that's when I got that same pop up. I guess with me just getting the 60K in November and then getting another 25K on my Blue Business Plus application in January it raised a red flag to them.
Usually I don't close any of my cards. I had to close one card in October 2020, an Apple Card, due to a technical issue but there was no SUB on it. I applied for the Apple card using the wrong Apple ID/ icloud account (I have 2 accounts) and I was told by Goldman Sachs that I would never get more than 1% on any purchases on that account. I closed it and reapplied using the correct icloud account. Other than that I haven't closed any accounts on my own since 2009.
Hopefully we both will become eligible for more Amex sign up offers soon! I'm definitely not gonna pay an annual fee for a card with a sign up offer if I am not eligible, especially the $550 fee on the Platinum. Best wishes to you when you decide to apply again!
@Aim_High wrote:
@W261w261 wrote:Always good to hear of experiences, thank you, although most of what I was thinking, was "Wow! How long did it take him to write that?"
I know, right, @W261w261? It was in a couple of sittings. I type fast but also known for my longer posts. Part of me putting that together was simply due to the surprise about the denial and trying to wrap my head around what they were considering. The list of card openings is something I imported from a Word file that lists my cards. I wanted to share it with the community so that others might learn from the data points but also for me to gain a better understanding from our members about where I had crossed the line with them. It appeared it wasn't anything to do with my AMEX cards. I don't frequent those other forums that Finstar referenced and I didn't recall seeing anything similar to this posted on My FICO, at least not lately. I thought it might be a good discussion topic.
Yes, I appreciated it @Aim_High because I don't read those others either and thought maybe I was the only one. As your your long post, I love your long posts and would have expected nothing less. 
Your analysis of Amex MR cards/Chase UR cards was why I leaned in and got the CSP when Chase sent me my preapproval in 2019.
I figure I may just need more time with Amex before they want to approve anything else. Since my husband got the Surpass, I'm back to considering getting a couple of the Chase cards I was thinking about sooner.
Thanks for the info, @Jordan23ww! This is type of details I was hoping other members would add about their own experiences with AMEX SUB denials. Yes, that's about one card a month from 12/19 until 01/21, so you were very busy! 
Congrats on that successful run and good luck to you also on the future SUB offerings. In time, I believe I will be eligible again but this was just unexpected and requires another adjustment to planning for new cards. Some known thresholds for AMEX would be nice so we could temper our expectations.
@Aim_High wrote:Thanks for the info, @Jordan23ww! This is type of details I was hoping other members would add about their own experiences with AMEX SUB denials. Yes, that's about one card a month from 12/19 until 01/21, so you were very busy!
You're very welcome! Happy to share! I definitely understand your shock! When I saw the pop up it was like a record scratch sound went off in my head. Lol! Thank you also for the congrats! My philosophy after not getting any new cards for 13 long years was I wanna get every card I can from my wish list and then I can chill. Other than the Amex Platinum and a Chase Ink Cash I can honestly say I have every card that was on my list. I won't be below 5/24 until August 2022 so unless I get targeted by Chase or become eligible for SUB on the Platinum I'm probably done for a while. Hopefully more people will continue to chime in and provide even more clarity regarding this!
@Remedios wrote:I don't think Amex has "tresholds"
When I was approved, I was about 9/24, with three cards being only two months old.
I've had pre-approvals for all of their personal cards since the day I created log in, I've gotten multiple offers for fee free Plan It, I got a promo offer for 0.99% for 9 months (as if!), and now I also have PL pre-approval.
I've also spent more than $35,000.00 (which is very little for that card, but all I needed, I have to feed other hungry cards) in a first year on Delta Reserve, so they probably don't care who else I'm seeing.
Being as data driven as they are, individual profiles are evaluated. I doubt there is any "treshold", or any internal rule on how much is too much.
If anything, I believe these are spend driven decisions.
If you got a good SUB on your gold, but your use has been sporadic, or less than they expected (indicating SUB chasing, true or not), they might be in a wait and watch mode.
Those accounts that were closed in a short time span aren't helping you any, either
In time and with regular use, you will be eligible again, though SUB might not be as appealing as is now.
Shortest period of time I've seen for someone going from "no sub" to "here is your sub" was three months, but it did involve significant spend compared to previous months.
Thanks for the input, @Remedios. I never thought of AMEX as being that spend-driven from what I had read, but you may have a point there. I put higher spend on it to get the SUB but then backed off. Still hundreds a month but not as much.
Yes, it's becoming evident to me that AMEX is a lot more sensitive to credit-seeking than I had thought but if they have "thresholds" at all they appear more complicated and customized than some other lenders. It makes them more difficult to predict, IMO.
I also have have preapprovals and I believe they would have approved the card if I had continued; just without the SUB, and why do that when you don't have to? 
I had never given those almost four-year old closed accounts a second thought until the denial, but I dug around and thought I would mention them for data points. It would be strange to me that they would look that far back on account histories, especially given the range of circumstances that surround opening and closing accounts beyond SUB offerings.
@Aim_High wrote:
@W261w261 wrote:Always good to hear of experiences, thank you, although most of what I was thinking, was "Wow! How long did it take him to write that?"
I know, right, @W261w261? It was in a couple of sittings. I type fast but also known for my longer posts. Part of me putting that together was simply due to the surprise about the denial and trying to wrap my head around what they were considering. The list of card openings is something I imported from a Word file that lists my cards. I wanted to share it with the community so that others might learn from the data points but also for me to gain a better understanding from our members about where I had crossed the line with them. It appeared it wasn't anything to do with my AMEX cards. I don't frequent those other forums that Finstar referenced and I didn't recall seeing anything similar to this posted on My FICO, at least not lately. I thought it might be a good discussion topic.
Most of the time I need to take notes when reading threads from @Aim_High as they are quite intense... @