Actually they grow in California as well but I don't think that is the answer you were looking for.
Geographic isolation would be my guess. Not all that uncommon actually, especially in the CP kingdom. VFTs only occur in NC and SC. The tepui of S. America have a bunch of unique species. Smae for many of the Highland Neps. They just get "stuck" in an area and are not "strong" enough to expand.
I know a lot of them don't expand there area because of a couple reasons.
1. it won't help them to be carnivorous in soils that have a lot of minerals, so they have to stay in bogs.
2.they just don't need to move , they are in a special part of an ecosystem with out the rest of it they can't live with out human help.
3. If they did move they would probly loose there carnivorous nature , so maybe some species could have already evolved out of the bog type setting.