I have yet to figure out why so many firearms manufacturers are in the northeast. It's unfriendly territory at best and I am almost certain operating costs would be cheaper in the South or Midwest.
Just a guess, but many of the bigger gun companies in the US exisited from the time of the civil war or before, the industrial might of the US at the time was in the NE. So at least fo the older companies that is just where they were. Over time certain types of companies will cluster for various reasons, aluminum plants in the NW and Buffalo do to hydroelectric power, guns in the NE due to an exisiting base of skilled crafstman, steel mills etc.
Note: Glock's US operations are in GA, and I would expect most new companies to be in right-work gun friendly locations.