When your score is generated, it goes through different scorecards. Each time a factor raises or lowers your score, the system remembers those reasons. They are then listed on your report. There will always be reasons but they may have had a very small effect.
When you have a reason listed of short history, it only means that your score was lowered compared to others that had a longer history. If you had a high score with lots of accounts, that difference could have been 1 point. I have seen reports with 800+ FICO where short history was listed as a factor. At that level, each negative factor had very little effect but they do list the top 3 or 4 factors.
As far as getting a CLI on existing accounts, I wouldn't waste any inquiries unless that account has been completeley paid off recently. You are not likely to get a new account with a 0% BT with your scores and account balances. I would zero out at least one balance and get the others under 80% util before you apply and ensure your FICO is above 700.
