I think it is because you have a high debt to loan ratio.
There are several ratios they use to determine eligibility and your credit score.
One is the ratio of what you owe and your potential lending capacity.
In other words.......if you have a credit balance of 2,000 on a 3k limit, ratio is 66 percent.
If you owed 2k on 20k limit, it would be 10 percent and for some reason that looks better to a lender.
Call your bank and see if you can get your limit raised.
I have no idea why you were declined, but this is just an idea.