Bacause back when Piccol (or however his name is spelled) first wrote Essentials, XP was the most recent RPGMaker. Making an Essentials game for a later RPGMaker involves porting it to a new programming language.
As for why it has to be a variation of RPGMaker, I dunno. That was a choice Piccol made.
To further elaborate on your answer, I believe Poccil had based his engine off the works of Flameguru, which was waayyyy back when RPG Maker XP was the most useful RPG Engine. (Debate me if I'm wrong, but back then I can argue with there being no other suitable RPG Engine for the job)
With that said, Flameguru made his starter kit on the latest and greatest RPG Game Engine. Poccil found it, but thought that it was subpar, so he took the liberty of making it more complete, and I *think* he worked on it for a solid 4 years? (correct me if I'm wrong) before Maruno took over development and Poccil disappeared into the depths of nowhere.
And this goes *way* back to like 2006.
So, the reason of why RPG Maker XP is the standard is because Pokemon Essentials is based on outdated software.
The reason nobody has made another starter kit that comes even close to Pokemon Essentials is because it's taken the kit 10 years (roughly) to get where it is today. Not only that, why would you spend so much time making your own engine when something that's already working to an undoubtedly impressive level?
If it ain't broke, don't fix it. If you cannot provide meaningful features, don't copy it. If you do, then you're essentially wasting your time as almost nobody wants to learn a new engine, with a small support team, little to no resources, ect. Compared to using something that they're already familiar with, has tons of support, and a lot of resources.
