Biggest reason? Familiarity breeds contempt. Everyone in this thread so far is North American, and exotic foreign species are inherintly more interesting to the average person. Many are also protected in at least part of their range. In combination, that means that very few enter the pet trade, so they don't get much chance to develop a following. Those of us keen enough to seek them out have to snap up the few species which do enter the trade, or go out and catch them ourselves. Given the small ranges [and the fact I'm in the plethodontid gap], catching them ourselves is often inconvenient at best.
I've worked with over a dozen plethodontids over the years, but currently have only P.chattahoochee and P.idahoensis.