They are rare because naturally there are just very few of them (as is true of any species). Add to that the fact that micranthum just does not breed very productively. Even the normal form of micranthum is problematic when it comes to breeding and pods (if/when they take) often do not produce many seeds.
This even more true of P. micranthum album and very few people have managed to get flasks of them. The few people I know of who were able to get flasks of them weren't able to get more than 30-50 seedlings out of an entire pod.
Albinistic forms of species are valuable because they are just so rare. A division of the original plant of P. stonei album sold for $25,000. This is the only known example of an albinstic stonei. The same is true of many other albinstic Paph. species.