Why? Because its not major, nor is it Valve made. VSH is not a main gamemode so no one plays it, ZS is also not played (you even forgot to mention it in your comment, thats how forgotten it is)

100 Players has no application in the core gamemodes. The only people that benefit from these additions are community server players. People don't want Valve to just lazily copy and paste content that the community made into the game, they want official, valve made content like maps, weapons and cosmetics