Thought I'd throw my hat in.

Why write Ranma?  Well, the characters are excellent archetypes.  You
can take Ranma in many, *many* different directions.  Admittedly that
stems from the fact that Rumiko left a lot to be fleshed out.  This is
why you don't see too many Maison Ikkoku fics:  that series was closed,
and the characters were fully explored.  Perhaps because Ranma is not 
the best of her works, people see a need to go back and give life to
some characters that might not have been given more than a cursory
glance.

There's also the unfinished aspect of Ranma.  This extends to Urusei
Yatsura as well, but that series is a) not widely available through
Viz, and b) it takes a while to get used to the main male character.
Ranma is a very popualr series, very open-ended. with a main male
character that most fans identify with in terms of his remarkable
ability to say the wrong thing, and want to be like in terms of his
strength and honor (oh, hush, Blade. ^_^)  Again, ideas for fanfics 
are bound to suggest themselves.

However, I haven't written Ranma in a long time.  I have a few ideas,
but none of them excite me at the moment.  And so we get to another
reason to write Ranma:  built-in responses.  There are a lot of people
out there who only read Ranma fics.  They would read and respond to
"Sour Times", for example, but might look at "Won't Get Fooled Again"
and think, "The Dirty Pair?  Evangelion?  Heinlein?  Too complex, I'll
give it a miss."  (Gee, Sean, cry us a river.)  (Yeah, yeah...)

(By the way, Ranma can be just as complex.  Thought I'd explain myself
as I go along.)

It feels good to get people writing back about your stories.  You get
that when you write Ranma.  You get it writing Sailor Moon.  And, since
it has a small but hardcore audience, you get it writing Bubblegum 
Crisis.  Every other series, you has to take your chances.

So, when people get lots of responses to their Ranma fics, they write
more.  Makes sense to me.

I personally agree that there are a lot of Ranma fics out there, and
that many of them repeat the same things.  However, I do not for one 
moment believe that the market is glutted and that reading Ranma fics
is a thankless, worthless task.  I will always be looking out for the
next Hearts of Ice, the next Chasing the Wind, the next Bitter End, or
the next Thy Outward Part.  Imagination is unlimited, folks.  And *any*
anime world can be taken wherever you want it to go, provided your
imagination is large enough.

--Sean Gaffney
--where's Erin Mills?  He could explain this better than I could...