That is a bit surprising, but lenders are still being pretty careful about their loans, to put it mildly, ever since the economy tanked in 2008. It may be that while your credit scores from the three major credit bureaus (Experian, Equifax and Transunion), are very good, and your credit history may be impeccable, your lender may feel that you're spread too thin. The way they might see it, a little more debt could mean a lot more trouble down the road.
So if I were you, I'd take a good look at your income, and the debt you're carrying--not just your credit cards, but your car payments, your mortgage or rent, and so on, and then ask yourself: Would you lend to you? Chances are, something is out of whack and that you're either carrying too much debt, or, frankly, you aren't making enough money. A lender knows that if they raise the limit of your credit availability, you may get yourself into more debt, and your credit card issuer may feel that you simply can't handle owing any more money. Of course, if you're making plenty of money, you may be right now thinking, "No, that isn't it."
In that case, you might also want to closely examine your credit report, which you can obtain for free three times a year at AnnualCreditReport.com. Perhaps you have credit card accounts open that you aren't even aware of. Let's say you have a Sears store credit card account open, and a Kohls, and you haven't used either since, say, 2003 (no knock intended on either store; I like 'em both), and you have an available balance of $500 on each, meaning that you have $1,000 in available credit. Well, you've forgotten all about them, but a lender hasn't, and they may be looking at the big picture and reasoning that if you went on a shopping spree and maxed out those cards and then, along with the other debt you have, maxed out on this credit increase that you want, they may see financial trouble. So something like that could trip you up.
Another suggestion, and it's just crazy enough that it might work. Call customer service at your credit card company and ask why you were denied a limit increase.