I know why we barely see urf in the year and one for all

It's because it's a balancing nightmare. I know some of you will say, "who cares? it's a mode for fun." But you do care. You care a lot actually, it's why you dodge, complain, cry, or just simply hate the idea of not choosing or getting the champ you want to play. It's when you get stomped by meta urf champs, it's when it's not as fun to play a champion that doesn't benefit much from urf and are fighting one that does. It's only up for a short period but were we to see it longer we would see things like "OMG NERF SIVIR ON URF, WE NEED URF SPECIFIC ITEMS, etc etc" they don't want to deal with that. Same thing with one for all, some comps work a lot better than others, some don't work together well. In any case this is why URF barely comes up a year and probably why we have ARURF instead of regular URF so people won't deal with the unbalance cancer as much and complain less.

TL;DR = Making urf or one for all fair play would be more work than balancing the default mode and the more regular people see it or play it the more they would expect some sort of balance to be done to it.