Can't edit on mobile.
So due to those factors a lot of people were sticking to shatter for their various frontline achievements because of how much a win offered you. It was also a victim of its own success.
A lot of people went there and the ones that didn't want to do the mode also didn't want to wait, so they kept queueing in to shatter. Until they release a new map or two with some different achievements/rewards to strive for, I don't see it's popularity diminishing any time soon.