Knobtail geckos are not easy to keep and breed, so they are still fairly rare. People have been known to spend sums of greater than $2,500 on a new leopard gecko morph or a rare gecko specimen, so I suppose the price tag alone would discourage all but the most serious, experienced, and committed collectors.
Marcia is right, generally speaking Nephrurus are not easy to not only keep, but reproduce healthy animals. On top of that, the genetic diversity is a lot more limited in the US compared to the leopard geckos and crested geckos out there. Plus supply and demand. Prices have fallen drastically across the board on them as well, you can find male specimens for less than $200 now on up to $7500/pair for N. asper.