>>2331220Consider that all the naruto doujins that don't suck are made by like 3 people. And one of those people are one of the two people that make one piece doujins that don't suck.
Consider that two two season animes (ore wa cute sister and k-on) are responsible for like 1/10th of 'parody' doujins. But fate/ with like 8 series and a dozen movies AND being based on an h-novel has a handful of doujins.
Popularity has no bearing on quality or quantity, it only increases the likelihood of an artist liking the series enough to make works for it, and for such an artist to be good enough to make works that are popular enough for that artist to take their efforts more seriously and potentially profit off of it.