You know that the engine garrysmod is running on have big (or should i say small?) limitations about everything, for example the entire Physics + Lua stuff runs on 1 core which will make big problems if you have more addons and entities at the same time (the networking stuff has its own core on the server)
S&box with Source2 will have more features (bigger maps and multicore support) and more modding abilities than Garrysmod (and a more clear programming language (c#) than Lua)
or in the language of this thread: this makes sense since using an over 10 year old engine in a over 10 year old game will decrease the fun of making addons for it over more time because even with very good computers and high end servers we cant make a big gamemodes (aka not very good optimised for current tech)