Because the money for UBI fundamentally has to come from the upper echelon of capitalists, and they don't want it because it both reduces their hoards of capital as well as relieves the structural stress on the working class that keeps them desperate enough to work for dogshit wages in order to continue surviving, and we do not live in a democracy that cares that 95% of the population would be better off for it because those same capitalists own all major forms of information dissemination, lobbying, and campaign funding.
Which just circles back to the core problem of UBI: if you somehow managed to unite enough people willing to buck the current system to get it instituted against the wishes of the ruling class, then you've also reached the threshold to end capitalism outright and bypass the half-step of making the ultra wealthy give back a tiny percentage of their ill-gotten gains in the form of UBI in favor of just building a society where the collective wealth generated by the workers is spread among the workers.