If you want to know why, take a look at ARAM - it was originally meant to be a fun, relaxed gamemode that turned into completely tryhard territory when people started making ARAM-specific accounts and only buying the champions that are OP there to increase their chances of getting said champions.
If gamemodes like URF were to become permanent, it would have been the EXACT same thing. Hell, it even started showing towards the end of the first iteration of URF and basically all throughout the second. After people figured out which champions were broken it was over and you saw the same champions over and over. If URF was to be permanent, it would slowly dissolve into another tryhard playground with different rules, just like ARAM.
In other words, blame the community for being tryhards in gamemodes that are meant to be fun.