Likely because welfare in general is unpopular and a subject of much antagonism. Particularly in the case of a Friedmanite negative income tax, it would create an unconditional basic income for people of lower income brackets, with all the moral and economic arguments there are against that.

Actually, a lot of libertarians would support basic income as a replacement for the traditional welfare systems and minimum wage laws. See http://www.libertarianism.org/columns/libertarian-case-basic...

Basic income guarantees have been advocated by many people in many centuries. One issue is that we don't know how they would work out in practice in modern societies, and the question is very controversial, which makes it hard to get good information on. This is some information that tells us that they might work out pretty well.

It's a lot older than that, and is also the normal practice for governments in most civilised societies: even in the US, there is a progressive tax system and various forms of tax credits and cash benefits to give low-income people more money than they would have otherwise had. That argument has been had in the past by people like Friedman and, at least to some extent, won.

What's more recent is that we've got a few long-term studies like this one providing results showing that no-strings cash benefits do give strong beneficial results. This was long suspected, but that suspicion is now supported by evidence.