It's a lot of factors. It was a lot of peoples' first 3D game experience, it's old enough that a lot of people who were enchanted by it in their childhood are

It's a lot of factors. It was a lot of peoples' first 3D game experience, it's old enough that a lot of people who were enchanted by it in their childhood are old enough to dig into it now, it's an early 3D game with relatively simple and primitive techniques that were state of the art for the time, and it's had a ton of time put into decompilation, and a few major leaks to help understand how everything fits together. It's also a very good game, and the controls and mechanics are more responsive and smooth than you'd expect for such an early 3D game. Mario games have all had a lot of effort put into understanding the way the code works under the hood. The NES Mario games have also been heavily broken down and analyzed, but there's not as much stuff you can do that's as exciting as what you can with Mario 64.