As Chana Joffe-Walt explains,
Part of Clinton’s welfare reform plan pushed states to get people on welfare into jobs, partly by making states pay a much larger share of welfare costs. The incentive seemed to work; the welfare rolls shrank. But not everyone who left welfare went to work.
A person on welfare costs a state money. That same resident on disability doesn’t cost the state a cent, because the federal government covers the entire bill for people on disability. So states can save money by shifting people from welfare to disability.