URF

Normal URF was a joke game mode for April Fools. It was well received from what people wrote about it. A lot of people wanted it back, a lot of people are frustrated and complain about the mode (as with everything). But there was a larger problem.

While Riot does want to bring URF back out for the people who want it, there is a problem with mode that has not been made public until recently which is the reason why it is so rarely brought back. URF made people stop playing League. People just quit the game in large amounts with Normal URF when it does come back. So Riot's trying other variations of URF in hopes that the cause is frustration, something that can be fixed like a dislike of frequency of picks or certain champions abilities or something like that and not a deeper seated issue.

If it also helps, here's the article explaining why certain motives such as why the normal URF mode isn't returning.

And it's been in many discussions before yours: why not add it more? Realistically that wouldn't be flexible for the game's health - not even close. Normally when URF comes to the game, there's enough new content that's come out with it that can be thoroughly tested and trialed upon (champions/reworks, items etc). If there were more; say 6 times a year, that would literally make 6 months collectively (for PBE users) to constantly play it out. Its novelty would more than absolutely die over time.