It's because the other maps needed reworks lmao.
Nuke - WAY TOO CT sided and graphics looked completely outdated (looks like 1.6)
Inferno - Retakes are WAY too difficult, banana pillars make common fights very inconsistent, closed off ceiling made retaking from apts difficult. Also, graphics were not amazing
Dust2 - Honestly didn't really need many changes (and didn't get many changes) so they just refreshed the graphics
Cache - A bit too T-sided, lack of CT aggression is bad considering the few spots CT had to play, so they added a couple spots and refreshed the graphics
Train - Too similar to 1.6 (Outdated graphics)
However, Mirage has great gameplay (many great ways for CT's to get aggressive, open bombsites allow for retakes to happen which forces T side to have utility after taking the site, there is a central portion of the map that requires great teamwork to hold onto, map also allows for individual high risk high reward plays) and honestly the graphics are pretty nice. It doesn't look like a super old map and already got a rework from its original 1.6 style version a long time ago.
Besides, I wouldn't hope for a new mirage any time soon. Changing the maps around too much is bad for the pro scene. They are already changing up vertigo so often and even Cache got a remake which in the pro scene will have noticeable effects. It's a bad idea to think they would make any mirage changes or removal anytime soon.
The map pool since Vertigo was released has been completely unstable.