Probably because of a lack of responsive and timely post launch support that saw a slew of game breaking bugs and problems including, but not limited to, piss poor net code problems around and for a few months after launch leading players to speculate Treyarch abd Activision introduced some new "skill based damage" system, the armory being completely broken for months after BO7 launched, the introduction of a slew of extremely out of place skins and cosmetics that absolutely killed any kind of immersion you could hope to get in the game, a number of absolutely broken guns and gameplay exploits, the controversies surrounding AI use in the game, the generally lacking post launch events and content, and a set of launch day maps that just really leaned waaayyy too heavily into close quarters combat and tight map design such that you'd rarely get anything but a chaotic Shipment/Nuketown small map experience that only got alleviated as new maps slowly got introduced to the pool and began to give us some actual variety in experience.
Like I really love BO6 myself, it may well be my favorite full package COD since as far back as BO3 (though Cold War is close), but it definitely had plenty of problems people, mostly, rightly bitched about and I can't completely blame them for dropping the game ultimately.