>Somehow there were more undeveloped areas

Everyone lived in suburbs that still had all the B/C/D rate lots undeveloped because it wasn't worth it. The country was still sprawling out, had mostly yet to pass zoning and other asinine regulation, etc, etc.

The incentive to cram industrial parks and office parks and housing into every nook and cranny of our towns and cities came later over the course of the 80s because regulation speculatively front-loaded compliance costs into construction and when that happens it makes more sense to develop a bunch of D rate lots already on roads and bulldoze starter homes and mobile homes already on road and utilities than incur all the "you'll need an engineered site plan for that, that'll be $50k" cost to proactively prevent problems that previously would have been addressed on an as needed basis after the fact where pertinent.

It's basically the same incentive structure you see with zoning wherein grandfathered in stuff goes up in value. Unless you've got some monstrously profitable project to justify the expense the numbers work better to buy out something that exists than to blow untold thousands fighting for permission or jumping through hoops to do greenfield development.