My son meets all the tests for a qualifying child. He is a 21yo full-time student, and we paid more than half of his expenses. Why does the $5,050 income limit apply?

The IRS site has the caution: "If your gross income was $5,050 or more, you usually can't be claimed as a dependent unless you are a qualifying child. " He seems to be a qualifying child, so why does TurboTax say he earned too much?