Why do I play other games than Fate and D&D?
(I'll explain why I modified the question lower down)
These two systems are go-to systems for me. If I think of a game I'd like to play, my first thought is "is it fantasy?" and if so, I default to D&D (actually 13th Age, but I'm calling that D&D for th purposes of this discussion). If it isn't, I default to Fate. The reason is that they can do what the standard genre tropes call for pretty well. D&D can cope with outrageously stereotypical Scottish dwarves, with Elric of Melnibone, with mad-science-built robots, with demons and magic, with Kender, with an evil inquisition, with a good inquisition, with Monty-Pythonesque inquisition. And the truly insane part is -- it can have them play not only with the same set of rules, but in the same campaign!
D&D is accepting of different styles and players in a way that few -- make even any -- other systems are. That's why 5E took a step away from 4E's much more focused play style and is more popular; it moved back to its traditional role as being highly adaptable and allowing a huge variety of anything fitting the genre of "fantasy". And that's a big segment of that players want to play.
Fate is my personal go-top game for, well, pretty much everything else. It also has a very broad set of assumptions and works well for a variety of styles of play. Unlike D&D though, it's not as good at mixing them within the same campaign. It's not as easy to mix and merge styles together. But still, it's a go-to choice.
So why so I play other games? For me, it is because another game hits a niche that I'd like to play. Modern day horror, for example. I could run it with Fate, but a modern-day horror-focused game will likely feel better for that niche; more evocative, more immersive. Or sometimes a game defines a niche -- Toon or Paranoia are good examples. A game can define a niche and then I play it because I want to play in that niche.
I added Fate to the OP's question, because the obvious answer is "I want to play something non-fantasy". It's that, true -- but there's more reason than just that, making it a go-to game for many.
