UBI will work even in an environment where many people have no marketable skills, while negative income tax would just be the state sponsoring activities with no real value.

We're not there yet, but the odds of ending up there sooner or later is very high.

(Negative income tax could still be good if the activities themselves have a positive social value.)

Negative income tax is just a more complicated form of the same thing. Part of the UBI appeal is easy administration.