I was born in the 80s and grew up in the 90s. My grandmother (yep!) had a Nintendo so I'd spend a bunch of time playing games with my cousins when we'd visit her. Eventually at home me and my brother received an SNES. The games on those two consoles were so fun and seared memories into my brain.
I think the nostalgia factor is huge for "retro" style games, at least for me. Any retro style game I immediately perk up and want to see what it's about. But that's not the only thing..because they still appeal to new generations and fan bases.
The art, while yes pixel, can still be impressive and beautiful. I think that's part of it: creating something amazing from basically 💩 building blocks. The same goes for the soundtracks.
And while the controls are limited, as Brian stated - the simplistic nature of the controls are actually a great boon. It expands the audience (age range and gaming experience) and lets the game developer focus on content and story.