Thats easy schmmeezy, assuming decisions should be slanted towards diversity (diversity in a game=good) then it should follow things that limit diversity should go away (keeping in mind making something go away is limiting diversity in itself). leftovers allows for more walls to be used, tactics like subcm and not to mention probably the entire stall playstyle while restricting very little if anything at all (is there a pokemon you can think of that you say to youself "oh darn i would have used him but too bad leftovers makes it unusable!" sr on the other hand limits a shitload of pokemon to lower tiers like charizard moltres articuno scyther while also adding redundancy to our game (blastoise is a bad suicune with rspin, we shouldn't use it except sr forces us to use it). letting blastoise be uu is not diversity since other pokemon would fill its role better, i shall name this pseudo diversity (which i contend is bad). you could argue that sr makes sure things like darmanitan counterable and keeps them in uu but then all you have to make is a cost benefit analysis - sr keeps many more pokemon out of any tier than lets one remain in a tier (specifically speaking about uu).