Because the law says that if he is age 24 or older and has income over $5200, he can't be claimed as a dependent. It's just how the rules work.
(If he is disabled, and unable to perform gainful work, and his income was earned in a sheltered workshop, he might qualify. That's more complex and we would need more details. But for a non-disabled person, the $5200 is a hard limit.)
