It's not surprising Mercs is the most popular, it's an open sandbox with tons of potential for moding. People into Battletech just really like making their own little unit and piddling around the inner sphere. If they double down on the successful Mercs formula they could really add tons on top of it to make it more immersive.
Not to say Clans was a bad idea, a story driven game was a decent change of pace from how procedural everything in Mercs was, but it didn't really capture the imagination of being a clan warrior. It was all stomping inner sphere trash hordes and not enough honor duels with other space psychos. Gameplay wise it was fine, but the upgrade system plus the dominance of laser spam got really old fast, but doing anything else felt like I was gimping my mech.
Online is a niche PvP game, it's fine for what it is, and it serves a crowd. Just not really everyone's cup of tea.