The biggest problem the game has is that once you get a fairly good collection of diamonds (let's say about 12 in total), the game's basically over. You really don't have any true incentive to progress further other than further increase your bragging rights.
Main story mode doesn't see any progress at all, any new origin story would be finished in at most a week by most people, prize fights are the same repetitive endless mode, which has seen next to no innovation whatsoever, dailies are tedious as shit to do, PvP still has frequent connection issues and doesn't have any real reward, and rifts are just a clusterfuck of "how much can I score on Sunday". I hope you like having your base recieve one or two attacks last minute.
The best thing to come out of SGM has definitely got to be the addition of Annie into the roster, and the possibility of new characters in the future. Still, most people will simply wait for her 2E release instead of playing her mobile counterpart.
There's simply nothing of interest to keep most people playing the game. That's why a lot of people just abandon it entirely; if you keep playing, either you REALLY are into SG to have an account into the game, or you probably spent cash in it so maybe you wanna make the investment worth it.