My guess would be simplicity and fairness: everyone gets $X per month, no matter who you are, that's a simple thing to get. It fits neatly into our pre-existi

My guess would be simplicity and fairness: everyone gets $X per month, no matter who you are, that's a simple thing to get. It fits neatly into our pre-existing mental models of how people get paid in the world. Tying it to taxes, sure you could still space it out monthly, but we're used to thinking of taxes as a once-yearly (or once-quarterly) event. Fairness: everyone, even millionaires, gets the same thing. The idea doesn't discriminate, so it's not just "the poors getting a handout" (n.b. I don't endorse that idea or phrasing).