I suspect that an important factor was that math was sufficiently abstract so that the smart people could go there and do their thing without risk of saying something that would offend the regime. If you said something carelessly in economy, you might have disagreed with Lenin. If you said something carelessly in biology, you might have disagreed with Lysenko. But in math, you could say anything you wanted, and you were okay. And in communism you were not allowed to start your own company anyway, which is why many smart people became math teachers instead.