I can't really speak for the rest of the state but, in the late 50's / early 60's, Huntsville's Redstone Arsenal army base ended up as the home of NASA's Marshall Space Flight Center, as well as many of the post WW2 German rocket scientists. I think that over the decades, the focus on aerospace & defense has simply snowballed as more employers and workers are attracted to the area.

Maybe some government decisionmakers decided to spend money there as a deliberate matter of social policy, others saw it as a good place to get certain property and services that the government wanted at low cost, and maybe some legislators were good at getting procurement and grant preferences for their home state?

Its like gravity. You make a divot in spacetime, and things being to roll downhill toward the divot. Mass collects, attracts other mass, and soon you have a significant planetoid. Leave it alone long enough, you have a star, or in the case of some places, a black hole.

In HSV case, several significant divots. NASA MSFC, Army Materiel Command (Logistics), and the Missile Defense Agency. Caveat on living here, if WWIII happens, we will get nuked.