URF does not get boring. People get bored of it because they don't know how to moderate themselves of a good thing. I personally (and many others would agree) could play nothing but urf for a full year and be far more satisfied with league than the rest of my experience with the game combined.
Likely they don't want to make it permanent because: it does not make as much money, people without self control(majority of the player base) get burnt out on it and play the game less, and it is a lot work to implement new champs/reworks/buff/nerf etc. All they would have to do is implement a pay-to-play system, one or two weekends a month you can spend xx rp to play og URF. I would gladly spend money on it and so would many others. I would also be far more satisfied with spending money on having fun over spending money on garbage visual flair. And to sweeten the deal as well as deter the "gets boring" aspect of it they could have the other game modes that we have been begging for aka OneForAll, Dominion, Nem Draft etc.