Price Tag and the amount of tools comes to mind when thinking why Rom Hacking is superior in popularity than Game Developing.
Imo, one of the key reasons why is inspiration. There are barely any fangames that people can actually play, and most of the ones that are available for play are either terrible, not a lot of work was put into it, terrible, not worth playing, terrible, extremely old, oh, and terrible.
If you go in the Rom Hacking forum, there are hundreds of hacks just waiting to be played by newcomers. And most are actually WORTH playing. The first hack/fangame I ever played was Pokemon Quartz, studied up on hacks for a while, not ever coming across "fangame" or "game devs".
Pokemon Fans that always dreamed of making their own can just play an entire library of Pokemon hacks and even though imo, it's harder to make a hack than a Game Dev, they have a bunch of tools, resources, and other hacks to look upon if they ever get stuck, need ideas, ect. I honestly cant say the same about the Game Dev department.