Odd. I use my Capital One cards only when I'm international and not on business (i.e. when I have to pay FOREX). It's been many months since I last used the card. I got some letter in the mail saying "Please use the card, call us and tell us what we can do." So I called and said raise the limit, lower the interest. Done and done. It's not the highest limit card in the wallet, but 20K certainly covers my foreign travel needs.

Now, the OP seems to have a nice interest rate. At capital one, I can only get a rate like that on a time limited promotional basis. Like the OP I don't run balances when there's interest. So perhaps the issue is they aren't willing to extend the limit as long the rate is so low.