Coming from mostly an ARAM player I find the normal SR to get boring. I find the reasoning behind that people would stop playing SR is ridiculous but look at other games like Smite they have a new game mode everyday why not have this for league?
You do know there's 3v3 and Dominion too right? I find Dominion a very nice change of pace from SR.
As for why league doesn't do it it has a large factor to do with Community Fracturing. In other words, adding a game mode will attract a portion of the other mode's regular playerbase. This decreases the amount of people doing them and increases que times, broadens the range of matchmaking pairs (more bronze vs gold situations and what not) and Riot would have to make sure none of those ques went below a certain amount of people, or else they simply wouldn't be able to match up well (it may just be the same people again and again). This actually isn't much of a problem in NA, there's a lot of us, but in other regions like Brazil the community is already really freaking small and adding another game mode may break it all. And of a game mode would be released just for the major regions that's denying content to the smaller server users and makes for very bad Public Opinion. The temporary game modes are a semi-good answer to this, as it may fracture the community but it's only temporary.
tl;dr Riot is global and it's smaller regions wouldn't be able to handle the split.
I think it's also because the game modes are cheesy. I mean, cheesy on purpose. Cheap thrills are their appeal. But for the average player, cheesiness isn't a satisfying long-term source of entertainment. Think about Mirror Mode.
The first time you see the enemy team aced by 5 cannon barrages, it's really cool and fun. But the 3rd time GP is voted as your mirror mode champ, your reaction is, "Not again! :S"
There's also the technical side of things: any new game mode, especially those that rely on cheese, are going to have something weird in them.
That leads to a bunch of design challenges, to make sure the game continues to make sense: Should akali be able to stealth in a friendly akali's shroud? What about an enemy akali's?
It also leads to a bunch of technical challenges, which means a bunch of bug fixing.
These things just take time, if they're going to give us quality content, which is why we can't have a new game mode every day, if you want them to stay as cool as they are.
One thing they could do would be to have the modes switch around every few days or each week
The popular one will be brought back - but how do they see if we like doom bots when everyone would still be playing URF right now?
If URF was permanent I would never go back to normal SR
