There are undoubtedly many reasons that add up to why there aren't many yuri stories.
For instance, if based strictly on canon events then the best you'll get out of Ranma is a heterosexual guy in mind and a significant other of the female persuasion who's either bisexual or forcing themselves to be for his and/or their sake (if Ranma were to actually feel incomplete/not loved enough unless he received the same treatment as a girl). Now, let's say that Ranma chooses to live as a girl, or is stuck as one, which ultimately means that he will be a girl in whatever relationship he has. You might expect him to still be interested in girls, which is highly supported and understandable, but that's only taking Ranma into consideration, who is just one of many pieces of the puzzle.
Is the girl he's interested in also interested in girls? Are any of the girls interested in him -- regardless of whom he prefers among them -- swing that way? How would his family, obligations and culture factor in? These things matter. A lot of people choose to be "in the closet" to avoid discrimination, persecution, ostracization, or worse. So, even if Ranma could find another girl who was interested, would they be willing to have a relationship and keep it hidden? Whether they hide it or not, are they prepared to bear with the repercussions/consequences if or when their relationship becomes known?
And then there's the fact that he's expected to end up with a girl, and there's so many to choose from. This and his curse can actually work against the creation of yuri content. It may not seem like it on the surface, but if the default pairing is with a girl, and being a girl doesn't change that, then what is there left to change? Exactly: switch out the girl for a guy. There are three main reasons why something like yuri happens, aside from the author's whim: it already exists in canon, there's enough to suggest the possibility (with or without anything strongly contradicting it), or it's exceptional. Due to both Ranma's character and his curse you could probably get away with the second one, but on the same basis the third one is also possible. What does that mean? Well, it's one thing if the character were always a girl and thought to be heterosexual, so it'd be exceptional to portray them as a lesbian. For Ranma, exceptional is when he's with a guy as a girl, and more so with a guy as a guy. Being with a girl as a girl really isn't exceptional, because his interest is still the same as expected, and the second thing mentioned before can blunt or completely remove the exceptional quality from the girl he ends up with.
Ultimately, what is written will likely depend on an author's personal interests and/or upbringing, but I'd like to think that the above and other such rational ideas come into play to some extent/degree or another, even if it's only to justify whatever irrational decisions that have already been made. As far as the topic creator is concerned, they should probably be glad that the supposed majority of female fan-fiction writers haven't gone on a gay-Ranma spree, because it would be so easy to justify having him getting in touch with his feminine side and being the uke to another guy's seme.