This is probably a sign of the times but MUDs seemed pretty...not exactly mainstream, but widely known in computing circles in the late 90s/early 00s when I started playing. They were one of the first things I stumbled on when searching very generally for online games as a teenager and were actually covered (very briefly) in a 100-level computer science course I took as a freshman in college. I was drawn to RPGs and the writing part more than the hack and slash part, so me finding MUs at that time seems kind of inevitable.
ETA: I stay because nothing else scratches the same RP fix and, for all my old friends who've cycled out, I meet new fun folks with decent regularity.