Story/Art style and gameplay.
Having simple gameplay is okay as long as it's challenging enough, a good example is Cuphead. The game play is very straight forward, but it's definitely challenging.
However if the game play is a little more complex like the Metroid games that's fine too, because they balance out the difficulty.
There's also games where the gameplay is dumbed down so much it's not as fun as previous games. For example Dissidia NT really sucked cuz because you would spend half your time playing on running up to your opponent. Not to mention the dumbed down fighting mechanics really made the game just less interesting in general. You didn't have either of these problems in the previous games. And it's not like they were too complicated they were just directional attacks and I think they kept some but they got rid of the variety of directional attacks. And it's not like directional attacks are too complex either because Super Smash Bros has that and little kids play it all the time.
Now when it comes to art style and story I think Cuphead is a good example cuz the story isn't anything like Lord of the rings, but it fits the art style perfectly. The creators for that game knew their target audience and they stuck with it. Instead of trying to please everyone they just decided to please a select group and so the game did really well.