The discrepancy between 2nd and 3rd are abysmal, I'm a avid fanfiction reader so I like to read some fanfiction from my favorite series.
I like Inuyasha and Ranma but it's more like, I watched and liked it but I don't identified with it, with Urusei Yatsura I go back watching some favorite episodes over and over again.
I'm kinda surprised with Maison Ikkoku though, with having such a low number of stories.
With the new series I hope that the numbers increase for Urusei Yatsura.
KasugaKyosuke15:
If I were to attribute it to anything, that might be because FF.net was in its peak right as the Inuyasha anime got very popular in the West. As for Ranma, a fanfiction community for it already existed on rec.arts.anime.creative in the 90's, giving it a headstart. I also think character ships are a big fuel for fanfics; If UY and Maison had love triangles, Ranma's got a love decagon. (Inuyasha's also got plenty ships to spare)
That is strange though... Like you said, here's hoping UY All-Stars boosts the numbers a bit.