Riot loses players to urf because urf is too fast compared to base league. When urf is over and people come back to normal games, it feels really slow and awkward; you're used to 2 second cooldowns, so 10 is now an eternity. You could have cast that 5 times over by now! That doesn't make urf inherently more fun, or base league bad. In fact, whenever urf rolls around, usually my friend group is burnt out of it by the second or third day. It's just too much, too fast.
Think about it like cake. You love eating a slice of cake every day and happily do so. However, you're given an opportunity to eat as much cake as you'd like for a week. After you're done stuffing yourself, you're probably gonna be sick of cake and not want it for a long while. Urf does the same thing.