It is one of the versions that lots of mods were made for, and so some significant mod packs have stayed with it for decades.

Part of it was the almost year between 1.7.x and 1.8, so some major mods like Thaumcraft never made the jump.

Gregtech New Horizons (GTNH) has been back porting mods and features to 1.7.10 (like the ability to run on Java 20+) because that’s easier than porting the whole mod pack forward.