CS just isn't as popular anymore. Quake was another big one. Things changed. We started to get games like Battlefield which had large maps, Halo was epic. Then CoD became the next hot thing, so other games tried to emulate that. And well, companies started to see how profitable that was. TF2 went f2p and opened up shop for making money with cosmetics and then it happened to CS: GO too. Now it's mainly battle royale stuff. People just want to jump into a quick match, get some kills or get killed, then hop in the next match. It's hard for multiplayer games to hold a person's attention because the market is saturated with a lot of things.
Not to say that there isn't a market for tactical shooters anymore, but the market has just changed.