Another issue is because there just isn't as big of an interest in scanning/translating the doujins themselves. I just looked up on Danbooru all of the doujin cover pages featuring Junko, and there are plenty of them, even discounting the hetero hentai ones.
While I do agree that there are less overall works about new characters compared to older ones, there are still plenty of new-character-focused doujins that simply haven't been scanlated.