The Theorem of Narrow Interests

The more constrained the thing you're looking for, the fewer good examples exist.

A corollary of Sturgeon's Law; as you add in requirements, the expected number of good examples of whatever it is you're looking for goes down.

Why this happens:

If you want Legolas fanfic, there's tons of it out there, and even though Sturgeon's Law says that 90% of it is going to be crap, the 10% that's good is findable. If you want a Legolas/X slash fic, there's still tons, but there are fewer tons, because it's a subset of Legolas fic, and the 10% that's any good is correspondingly smaller. If you want Legolas/Draco slash fic, you've moved into a still smaller subset, by adding in a crossover requirement, with a still smaller "10% good". And so on...

A second factor that also contributes is that writers tend to stay in the same general area for most of their work. In other words, a fanfic writer who has done one Legolas/Draco-is-a-crossdresser BDSM fic is more likely to do it again. This means that one writer may be responsible for a sizable chunk of all the stories with a certain narrow set of parameters, and if they're a bad writer, they're going to skew the crap-to-not-crap ratio (and groups like the PPC are going to have a field day.)

The porn version is closely related: Vampire/fetish-latex porn is rarer, and thus has fewer good examples, than either just vampire porn or fetish-latex porn.

It doesn't simply apply to porn and fanfic, though. Any narrow-interest niche is going to have a smaller overall selection than a wider niche. Thus, if you want to see a movie about World War II, you're spoiled for choice, but if you want an Alternate History World War II film where Those Wacky Nazis won the race for the atomic bomb by making a literal Deal with the Devil, you've cut your options significantly.

Also known as "Beggars Can't Be Choosers". Additionally, it has sadly unfortunate consequences for those who would plagiarize the work of other fanfic authors, as they almost always select works to claim as their own that will be immediately identified by the very people they're trying to impress with their "original writing".