Some species make amazingly interesting, very long lived, and not very secretive captives. I've kept 21 species between the zoo and my home collections, and for me they are way more interesting than any Ambystoma or newt I've kept. I'd have to say the number one reason they aren't very popular as captives is that there is not money to be made selling them in the U.S. Laws preclude many native species from interstate commerce and wholesale trade in them, and few people care enough about the species to actually learn where they live, go there, and legally collect them. On top of this, many are not robust enough to survive the physical abuse of commercial exploitation or newbie keepers so those that do end up making it to the pet trade end up dying.
With proper care many of these species will thrive and breed readily. If a fraction of the effort that people put into dart frogs was directed towards plethodontids they would be thriving in captive collections. However the little thumbnail frogs which are every bit, if not more delicate and secretive can be sold for $300 and up, while collection of many of the most colorful or interesting plethodontids is prohibited for commercial exploitation.
That being said, just like dart frogs and many other U.S. herps, there is a huge market for them in Europe once they are smuggled (or legally exported?) beyond the reach of their native country's laws.