>>279882355Doujins take too much effort, so you can't draw just because you like something. They have to be about a popular franchise, include popular girls, and be popular among the crowd that buys ero-doujinshi. For example, Spy x Family gets doujins precisely because it's something everyone recognizes, and everyone wants to bang Yor (and in some cases Anya). The same goes for something like MHA or Konosuba. Franchises have to prove themselves before they start getting doujins.
On the other hand, gacha games are perfect for doujins - they have gigantic casts of girls, they're popular with the otaku, and they're evergreen. A Blue Archive doujin is guaranteed to sell to a very large crowd.