My husband has 100% disability through the VA. Why doesn't he qualify for the disability credit?

VA benefits are not taxable and you do not enter them on a tax return.

For disabled adults:

There is an extra amount added to your standard deduction if you are legally blind.   Your federal return does not provide any other credits or deductions for saying you are disabled, with one possible exception.  

There is a credit for the Elderly and Disabled which is so small that very few people benefit from it—-it has not been updated/increased by Congress for decades.

ELDERLY OR DISABLED CREDIT https://www.irs.gov/help/ita/do-i-qualify-for-the-credit-for-the-elderly-or-disabled

There are some states that provide various credits to elderly/disabled folks, so watch for that when you prepare your state return.   The states that I know of that have anything for elderly/disabled are AZ, CT, DE, IA, MO, ND, NJ, NY, VE.   There may be others.