Several reasons:
The RPG Maker XP price ($30).
The ROM hacks have a very better compatibility (of corse, not all ROMs can be fully played in all emulators)
Game Hacking have some features that the main starter kit (Essentials) doesn't have (yet) like Contest/Animations, but in Essentials is very easy to do some events/custom systens and even add 500 big maps with a huge number of tilesets, put 1000 custom pokémon/trainers.
And...
Because this forum is the center of the Pokemon ROM Hacking community, at least for English speakers. While PC could also be claimed as the home of Pokemon Essentials, the ROM Hacking community existed long before Essentials, and it's taken years to build up this much of a community at all. At one point this forum was buried deep and it got very little exposure to PC's member base at large - and that remains a problem to this day. Plus, proper game development requires a much greater investment, and a lot of those who find themselves involved in PC's ROM Hacking community are kids looking for a hobby that they don't have to spend much time on.
So, if anything, I blame the school system.
Exact. I think that the main point is: The gamers know about hacks (like Mario, Sonic, Megaman, Pokémon, etc...) very more that the existing starter kits (specially Essentials who doesn't has a divulgation that it deserves) and theirs features.
That plus the fact that (on my opinion) people here are too perfectionist and pretty much never release even a small demo per say. No games to play equals no popularity, that equals less and less people interested in joining the Game Dev community since it looks like people can't get anything out of it.
This is a generally non-profit game development problem. If you aren't too perfectionist, few people will support you and you end up giving up. If you are too perfectionist, you game will take years and you/someone in your team eventually become bored and give up.
Let me tell you all a little story:
Years ago, I was looking to find a way to play Pokemon games on my pc.
When I found out about rom-hacking that way. At first I wanted just to edit a few small things. Like making certain Pokemon more powerful, or allowing to find wild Totodile on route 1. I then realized it was possible not just edit a few little things, but to create completely new game altogether. And that's how I found out about ROM hacking.
I think my story applies a lot of other people out there too... Some people may not have the intention to make Pokemon games at first, they just stumble upon it like I did.
This is important too. In ROM Hacking is fast to edit a game to include all pokémon only changing the encounters and putting some overworld that can be battled pokémon and NPCs that give the pokémon or even change trainers parties, but in Game Dev you need to create a entire game that may sounds intimidating. Theses small change are really effective for beginners.
Maybe it's because you barely have to learn anything to start making a game with RMXP, whereas you need to learn stuff before you even get started with ROM hacking. It therefore comes as more of a hurdle when you want to do something more than the absolute basics in RMXP, and it puts people off who had just gotten the idea that it was easy. With ROM hacking, you have the mindset that you'll need to learn things from the very beginning, so learning a bit more isn't such a shock.
That plus the utter lack of completed games to inspire people. The fact that Essentials is only an approximation of the Pokémon games may be another factor.
I'm uncertain about the first paragraph, but the lack of successful projects/games I believe that is a reason. Good games/devs lures people, similar to good singers/actors, and these people may will make/be good games/devs. Success is like a snow ball.