Your initial point was content. There's absolutely no content being introduced in CS since it's inception. Progression is as basic as it gets. People actually don't like other maps. They just want to play the same ones over and over.Because you cant get the game on consoles, and its only available on pc. Its because you can make money from csgo. Its because its franchise that have been updated for a long time.
People play wow, despite the game coming out in 2004. If people are attached to something, and it gets updated for a long time, they will stick with that ship, and not jump from it. Same thing for league of legend. Halo simply doesnt have that power right now.
Halo is missing the old features people used to love. Not to mention, they are games, which halo fans are occupied with. Halo is an old relic at this point, not because people dont like it, but because it doesnt get well in to the new era. It doesnt have the trade system that csgo has. It doesnt have BR mode. It doesnt have alot of units like league of legends, which you can bond with. It will need time to regain its spot, but as of now, they really dont have anything compelling like CSgo. Their online mode is just regular thing xbox players enjoyed for a long time. The masses dont care about that.
You said CS and LoL are only on PC. Then explain why Apex is doing so much better? What content excuse. People still mostly play the same map with their prefered hero over and over. Lack of content excuse is the most overused excuse ever. The reality is much simpler - the game either hooks people or not, regardless of how repeatative and content dry it is. CS is the most repeatative gameplay you can imagine but people look past it because they are hooked and love it. Meanwhile Evolve dies. Sea of Thieves content is still considered shallow by many. It's just chest runs and skeleton killing over and over. But people are still hooked to that repeating loop.
You are doing mental gymnastics when the answer for why Halo is dying is much simpler - people just don't like it anymore.