Why DarkRP & Starwars/Halo the only ones that seem to have a thriving community left

Well, in my opinion this effect have 3 major causes.

first:

Gmod is a bit old game, the source engine provides a too oldated gameplay and graphics, the maps are smalls and with heavy limitations and the game is too unstable... So the majority of the players who actually started to play gmod thanks for some youtube videos from 2013-2014 droped after some months of gameplay... gradually theres less and less active players in general...

Second:

The multiplayer community can be a bit cancerous... as you said in the options, some big servers with custom contents are full of drama... and some of then provides a too monetized gameplay... this is a bit toxic for the players, so it gradually kills the multiplayer.

Third (The biggest one):

THE SERVER LIST, dammit i just hate this server list, showing the gamemodes with more players kills smaller gamemodes... since they implemented this crap the "big four" (DarkRP, TTT, Muder and Sandbox) completely dominated the gamemodes lists...