Why don't we qualify for the dependent care credit?

I earned approximately 250k this year and my wife is a full time student. The reason given is that: "Since your $15,080 in care expenses wasn't enough to use up the $4,246 you contributed to your employer's dependent care plan, your Child and Dependent Care Credit is $0." I'm at a loss. Other forums say we both need income, but she's a full time student - which should qualify even though she didn't have earned income. I'm at a loss.