It's very simple.
That's the answer - the simplicity. A lot of new games have a huge number of variables, powers, loadouts, specific maneuvers, special powers, weapon spawns, game modes, and a huge number of very specific game systems.
Counter Strike is an extremely simple shooter - each team has one objective and starts with (nearly) the same equipment and team size doesn't matter. You can play 2v2 or 10v10. No "roles" (no tanks, no supports, no synergies), no shifting meta, nothing locked behind a cash shop - you can just hit "Play" and play the fucking game. It's something that a lot of newer developers overlook when designing an "e-sports" game, that making a game that's easily accessible (as in, your ability to actually play it) will be successful.
The simplicity and accessibility also extends to the weapons and characters - real world skins, real world weapons and no "diversity".