Why does Riot rotate the fun game modes instead of keeping them all?

I don't understand why Riot rotates the fun game modes and why they don't allow us to play them at all sometimes.

Why doesn't Riot just keep them all active so we can play them anytime as well as have a choice of what game mode we feel like playing?

I heard people say that some people ruin URF by making it less about fun and more about winning so Riot takes it away to keep it fresh and just about fun. Some say it's so that we don't get board with the game modes. If bugs or balance is the issue, then why not just update it with new patches like normal?

In my opinion, Riot should just keep them up and leave the decision on when and how we play them up to us. I mean, DotA (1 and 2) has allowed that to happen for years and the world hasn't ended! So what's Riot's reason behind not letting us play freely?