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.