Main issue that I've seen is server fracturing. Smaller servers/regions already suffer from not having enough players to adequately support matchmaking. Temporary modes are fine because in the end they go away, a new permanent game mode however would take players away from the other ques and cause even more numbers issues.
Of course then there's the argument of "why not just do it for the servers that can handle it?", and then you get to flak from accusations of exclusivity, catering to certain regions, yada yada.
ARAM didn't run into these issues because it was already a game mode made by players through customs and simply proved it could be a que already.