Several reasons, in descending order of importance:

1. Cliff effects. If you receive $1000/month while unemployed, and you have the opportunity to take a job that pays $1250/month, the actual delta is now only $250, which may not exceed your threshold. A basic income doesn't go away when you get a job.

2. Administrative costs. Figuring out who needs what costs money, can be gamed, and creates bad political incentives (pandering either to the group that wants more welfare, or the group that's upset about paying for peoples welfare).

3. Fairness. Some people feel that it is unfair that the poor receive monetary benefits from the government just because they are poor. If everyone gets the same benefit, unconditionally, that may be more palatable to this group of people.