Pumpkins grow really well in a lot of places, as long as you aren't interested in having them perfectly timed for Halloween.

I imagine that in most states, pumpkin production is aimed at local-ish Halloween demand for both decorative and pie purposes. The market is likely not large enough to support a bunch of canning factories like this, and there is a chicken v egg problem for moving the factory somewhere else, ergo this place lives on as the pumpkin capital.