My response is directed to your post's title and a bit to your description. And to just add to that, ARAM was a long time game mode, and highly popular to become a permanent one. ARSR so far has had more critics and negative feedback than anything. And for what? It's lack of creativity? It's copy-paste formula? I feel that's more of a player problem than it is Riot's.
You know they had Nemesis Draft at one point in time right? You know...that game mode where your enemy team chose your team and you played a game just as normal as that? Such a simple concept as well. The only difference between these two specifically is how uncommon it is that customs even do this; which literally answers your question as to why it's in rotation in my opinion. With the reception ARSR has gotten, it's better off as a RGM because of that.