Probably because a large number of modern roths come from only a few jungle collected plants
Many rothschildianum have been wild collected over the decades, even today, but few will survive to bloom.
Nearly all the albino paphs that are in cultivation came out of 2 things:
- wild collected individual, that's maybe 95% of all the albinos known.
- or selfings of a plant, that's how we got the albinos of hennissianum, fowlei, lunatum, and possibly kolopakingii.
sukhakhulii album came from the selfing of a wild collected aureum. There has never been a wild sukhakhulii album found until today.
Millions of Paph delenatii were produced over the decades, none ever bloomed albino. The albinos all came from one colony in Viet Nam that had a couple dozen alba plants. They are the origin of all the alba delenatii. On the other side, the 'dunkel' form of delenatii was an entire individual, separate colony.
To get albino roths would require selfing the 'right' coloratum plant that has a genetic defect responsible for albinism, whilst being impossible to see visually. And the trend for roths is more to cross different individuals together. As for wild roths, I saw at least on 3 occasions pictures of very light color/green stems on roths in the wild. One is around too on Flickr. Those plants have never been bloomed.