As KitsuneFreak said in this thread, a large part of it is the company you find and keep while playing the game. There's also factors of getting what you want out of the game, looking at the master server list clearly conveys that Coop has always remained the biggest gamemode for many to play, others going ignored and not often hosted with enough players needed to keep them going with a more direct multiplayer basis compared to coop where having even 1 person is inconsequential. Thankfully the modding community is quite sizable, and I've found many ports of mods in 2.1 making their way fast into 2.2, and hopefully more big mods in the future. And if the idea of exploring mods does interest you, the 2.1 library has a lot to offer if you and some friends may be interested in that before some larger mods (such as the SUGOI, SF94, or others) make their way in.

Of course aside from all this there are still the other mods to try and enjoy if you haven't already, from the consistently popular SRB2kart, into some other mods like SRB2 Top Down with a unique style of play to try out.

And with the playerbase, as also covered before, there are people of all ages popping on, a lot of the older players aren't online in public games nearly as much anymore due to life circumstances and general feelings of having their mileage in the game spent aside from maybe checking out the large updates now and again. Overall I do hope you can still find ways to enjoy the game, experiment, or even take up learning some mod making yourself if you would like to bide your time that way.