Because maintaining the game modes takes a lot of server data and what not. Also a lot of bugs will need to be fixed, balancing the champions and making sure the game mode doesn't crash with millions of players queuing up every second.
Imagine you have a favorite box, you want to place everything you own in it. You can't because the box simply isn't big enough. If you put too much in, it'll break. If you put too little in, you invested in too big of a box. You want your money's worth so you want to make sure you paid for a box that can store everything yet doesn't need you to fix or cut the box to size. In Riot's perspective, they want money and they want to make sure their main game modes are being pushed out and advertised/played on more.
I'm with you though. Riot fucking said they will do a rotation but fucking lied and now it's all Poro King or fucking some dumb ass game mode that doesn't make sense. (Nexus Blitz and that other dumb retarded one)