This are just my personal opinion that should be treated as a grain of salt rather than taken as it is.
1. It is evidence that moege is actually quite popular, despite how a part of the vn community thinks that it is just a lump of diabetes. (I mean look at Nekopara)
2. Because moege has simple story. Meaning, it is easier to translate. Hence, for translators who are still lacking in translating experience and would like to get that experience, moege is a very tempting choice, rather they can't go wrong with it. (I mean you can't expect a rookie translator to translate something like Dies Irae right?)
3. Just general assumption, most fan translation are made by translators who are still new because they want to get experience. We should expect larger portion of veteran translator to not be involved in fan translation projects because they want to get paid jobs because they want to make a living.