Another consideration is that a lot of hardware startups focus on industrial applications with customers that don't operate out of tech hubs.

There are tons of small companies making hardware for oil & gas applications, but you'll mostly find them in areas where their customers are located (e.g. Texas, Alberta) and/or LCOL cities where you can find cheap real estate. This also applies to other industries like agriculture and automotive.

These aren't "sexy" companies so you don't hear much about them, but I'd wager that if you add them up, they'd outnumber the number of hardware startups in NYC and the Bay Area.