I’m not sure what platform the OP is on, but I’ll address xbox.
The main thing that hurt Brink’s popularity on xbox was the bad launch. Single player, back then, meant playing against super deadly enemy bots, while the bots on your own team were only slightly better than useless. Multiplayer, where Brink really shines, was so laggy that playing online became an exercise in frustration.
The other thing is that Brink just isn’t for everyone. It’s an objective based team game, with no real reward for going off on your own and racking up kills. It also doesn’t track stats that CoD players tend to value, like K/D.
Finally, many people got frustrated by the relatively inaccurate weapons. Still the best online versus FPS for me though.