This is the answer. Disconnect the economic floor from the means to fund it - when you have a NIT, it tightly couples its revenue source to itself. One of the cornerstones of UBI proposals is to tie it to cost of living to avoid having to touch it, whereas with tax code you often want to change it all the time determined by how prosperous or stagnant the economy is.

In addition, humans are kinda weird animals. It is much easier to pitch much higher taxes and a guaranteed $1500 a month from the state to people rather than higher taxes and no state check.