In the 80s, when I spent A LOT of time gaming, my friends and I just tired of it and we play many different systems, including those we made up ourselves.
Now, 5e is my main game and I just don't have much time to game. I do still play other systems, but generally as one shots.
At conventions, I like to try games I've never played, just for the novelty and variety.
I run the Expanse because I like the hard-science theme paired with the easy to run and cinematic AGE system that the Expanse is built on.
I run Paranoia because it is the only player v. player TTRPG that I've found to be consistently fun and because I love the flavorful fluff of its humorously distopic setting.
I play and hope to run soon Dialect, because it is the most consistently moving and meaning experience I've had with a TTRPG.
I play and run InSPECTres when I want to enjoy beer and pretzels party-type game with a group of extroverts who aren't afraid to let their inner thespians out.
I play Pathfinder when that's what the DM is running (no interest in running it).
I play Dungeon Crawl Classics and Mutant Crawl Classics when I want an OSR-with-a-twist feel. Never ran either, probably won't, but have enjoyed playing them and browsing the books.
I'm sure that there are more games that I would love playing. But I'm a father and at the peak point in a highly-demanding career and have other interests than just TTRPGs. So, when I do try new games, it is usually because someone else it running it as a one shot, usually at the one convention that I go to each year.