Both, I guess, because they're correlated. Certainly culture plays a big part. The USSR days were intense for science and nationalism. I guess those books were just another reflex of the space race mentality that stuck for a while.

I don't remember which Mir books I've read exactly, but I'm almost sure two of them were Irodov's Fundamental Laws of Mechanics and Saveliev's General Course. Maybe I was just too immature, but that thing was way too heavy for a standard high-schooler.