The reason few people breed pure species has been stated: Nepenthes hobbyists are few and far between, and Nepenthes themselves can be some of the most demanding plants most people ever own. You take this into perspective and the fact that any of the few large collectors and breeders in the world ever even manage to have two flowering male and female plants of the SAME species at the same time is a miracle in of itself.
Also has been stated, but Nepenthes are true tropical plants. As with most trees and plants that grow in the tropics they often flower in irregular patterns or whenever the often very specific sub climates that various Nepenthes species grow in allow them to do so.
Also I think it should be stated that in comparison with Sarracenias, Nepenthes are often larger, longer lived plants. In all reality they are extremely different plants all around.
And you ask what keeps Nepenthes surviving in the wild and not getting interbred into oblivion? Well they do interbreed quite regularly, as there are many known naturally occurring hybrids. What keeps species from disappearing altogether is that the islands they are endemic to separate species from each other geographically. (many species secluded to single peaks, basins, or cliff edges.)
The irregular tropical flowering season is another reason why many Nepenthes species ARE rare, vanishing or endangered. When man interferes with environmental factors it can severely disrupt the delicate breeding cycle of these awesome and complex plants.