Here is the reason: ASCII was tired of lost profits on the illegal versions that were translated to English, so they made the official English version.
Here is the entire story in a nutshell (PLEASE NOTE! I do not remember the entire story, so some parts may be wrong or left out.):
ASCII releases RPG Maker 95. Someone translates it and releases it on the internet. It wasn't a big deal, because it wasn't as popular. ASCII ignores it, and eventually releases RPG Maker 2000. Don Miguel translates a huge part of the program, and released it. The impact of 2000 was much larger than 95, because you could make console-like RPGs, not PC ones. So ASCII took it to court and Don Miguel was forced to stop distributing the program and if he didn't, he would be charged a hefty fine. ASCII though this would scare off anyone else who tried to translate RPG Maker 2000. So he stops, and another translation group picks up where Don left, and they distribute the translation, but anonymously to prevent ASCII finding out who it was. So ASCII releases RPG Maker 2003, and once again, someone illegally translates it. These translators, I believe, were also anonymous. So ASCII threatens with a lawsuit, and the translaters stop distributing it, but RPG Maker 2003 already has its English user-base, so the program could be found almost anywhere. Then comes RPG Maker XP. There biggest RPG Maker yet. This supposedly outsells RPG Maker 2000 in Japan, making it the most popular yet. Then two translators/translation groups translate the program. It is either distributed as RPG Maker XP Translation or RPG Maker XP Postality Knight's Edition. ASCII knew this was going to happen sooner or later, so they planned on releasing an English version of the program. Unfortunately, the translated versions are released way before the public even knows about the genuine English version. So, Enterbrain eventually releases the information about RPG Maker XP in English, then eventually the program. For there next RPG Maker program, both the Japanese version and the English version will probably be released side-by-side, as to prevent more illegal translations.