And where the people were; they didn’t need masses of soldiers to defend empty wilderness after all. Or keep the bears from staging an uprising.
But yes, putting armories in cities is a practice that originated long before travel became as fast and easy as it is now. I think it should be kept in mind that building armories in cities is a practice that predates gunpowder, much less modern transportation. People a thousand years ago were thinking in terms of “we need to arm guys with crossbows and spears to defend the city walls”, not modern mobile warfare.
So I expect a major reason they were built in cities, is because that’s where armories had always been built.
The few armouries I have been in, have been what would be called, in Australia, “a drill hall”.
Drill halls are built in places were people are. Universities, cities. So that people can get to them for drill. I have no doubt that there was also an arms depot / arsenal associated with the drill hall, but I’m not sure that was the main purpose.
The old armory in St. Louis was built at the intersection of a major north-south and east-west route. When it wasn’t being used as a “drill hall” (it’s where my brother-in-law went when he was in the Army Reserve) it was filled with jeeps, trucks and other equipment that the National Guard would need in case of a disaster. Not so much actual weapons AFAIK.
Most old armories I have seen in the Boston area have been turned into condos or office suites with absurdly thick walls.
The Salem armory is interesting. The front half of it burned down, but they’ve retained the original entrance, out in the open. It serves as a gateway to the park that burned-out portion became. It’s across the street from the Peabody Essex Museum, and is a nice lace to sit while touring the city or shopping in the area. The back half is still intact, and is used as the Park Service as a museum of Old Salem (and to remind people that there’s a LOT more to Salem than the witches). It’s also got a couple of public rest rooms that are cleaner than the ones in the Salem Mall next door.
The Salem armory also used to be the home of the National Guard band, which was renowned for its skill and for the original compositions they produced. The band used to perform at the Wonderland amusement park back in 1906-1910.
Troy NY Armory adjacent to the RPI campus (might be part of it now?) used as an event facility. Back in 1967 (could have been spring 68) saw Jimi Hendrix there. Marvin and I “borrowed” one of the advertising billboards (plywood about 6’x 4’). Very colorful with a $50 reward for returning it. Nah. Marvin had it displayed in his room for a while. Kind of became a ‘hot’ property. Transferred to my room used as support for the mattress. Probably still in a storage closet somewhere in the freshmen dorm. Good times.
At a guess, I’d say that as the land they were sitting on got more valuable, and it became easier for Guardsmen (and -women) to drive to the outskirts for their training & assembling, newer armories were built outside of the inner cities and old ones sold for real estate development. Makes some sense and allows legislators to avoid unpopular tax hikes by banking revenue from the real estate sale. (It would be interesting to study the effect of such movement on the psychology of the soldiers with regard to the people who live in those inner cities…)
Another aspect of it is that those older armories were built before the era of mechanized (or even motorized) warfare. So while they might have enough space for small arms and artillery pieces, they wouldn’t have enough land area for the truck (and track) park of a typical modern infantry battalion - and in the city, it would be expensive to get more space.
A lot of the older, pre-auto era armories were built very solidly - almost fortifications of their own - which may contribute to why they tend to be repurposed rather than razed.