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.

That's the thing; Filler episodes usually have a negative connotation for anime fans, but I find Urusei Yatsura to be the exception to that rule. "Dimensional Switch, Where's Darling!?" is one of my favorite episodes of the show, and apparently has a high rating among Japanese fans as well... That ending gets me every time.
I agree "And Then There Were None" is another memorable episode.

Just found this video. Someone used the new opening and put the "Lum no Love Song" music. Funny how it lines up (for the most part)!! Not sure if coincidence or edited to fit that way tho 

https://www.youtube.com/watch?v=yfJ1LcNhlRo

<a href="http://www.youtube.com/watch?v=yfJ1LcNhlRo" target="_blank">http://www.youtube.com/watch?v=yfJ1LcNhlRo</a>

My opinion of Episode 3? BRILLIANT! The introduction of Mendou, Ataru's election, the trip to the cave...wonderful. Really captured the comedic flavor of UY. In fact, this is the first show in a long time where I actually laughed over and over again. To say nothing of the post-credits sequence.

I especially enjoy seeing more of Ataru being not just a fall guy or oaf. To have his classmates notice that Ataru has strengths and value. He's a much more complex guy here. That definitely helps me appreciate him a bit more in having a coveted relationship with Lum-chan.

I really need to do my research on the original anime. I've been a fan for over 18 years but never read the manga nor did research on what's original content or anime exclusive.

I fail as a fan!