$819 balance on a $850 limit card.
This was more than likely the reason.
You are applying for a charge card that they expect to be paid in full each month, but you have this card reporting at almost 100%. They only see what is reported at the time, not what you normally do month to month.
Give it 30 days make sure all cards you have are reporting below 8.9 if not zero and try again. I believe you will see different results.
Sorry to hear about the denial OP. As many have already said, it was probably the card that reported a nearly maxed out balance that concerned them. Allow 30 days to cycle in case your request for reconsideration does not work out. Leave a small balance to report and then prior to applying, hit that pre qualification tool available on their website and if you qualify for any of their Charge Cards, then you are likely to get a more positive outcome on the Platinum application next go round. Wishing you luck.
You gotta be a little more patient you little whipper snapper! : )
I am not the most patient either and this is a fun game to play BUT you need to be more strategic about applying. Make sure your reports are good and utilization is way down BEFORE applying. It sucks to click "Apply" and get an immediate denial! : (