1. it cost money from those of us who work, the idea offends peoples sense of fairness.

2. the reduced bureaucratic hurdles to get the money would make it way to easy for people to just coast on the dole, econ speak it provides a disincentive to work.

3. Current welfare programs incentiveise good behavior, you have to buy food to feed the kids with food stamps, you have to look for work to get unemployment. Just giving people money will let them buy drugs gasp!!!

4. Many conservatives oppose all welfare programs regardless of their Cost/Benefits. By libertarian philosophy this is just plain old robbery.

5. Public employee unions would lose a lot of workers that currently pay dues if the welfare state was stripped down to just 1 program. Current bureaucracies in the welfare system give push back to these kinds of programs. Every program in existence has some public representative that brought it to and ensures it's existence (Senators pet project kind of thing).

I'm sure there are other things that would stand in the way of this kind of program.