It was a combination of factors. The Soviet Union had established itself as an alternative to capitalist predation, so the "welfare state" began to make sense as a concession from capitalists. Trade unionists and communist organizers on the ground risked revolution, so there was direct incentive to implement it.

Also, it came at the expense of marginalized peoples. Not only did many people not see the benefits domestically, but it was propped up by pilfering surplus value from the global south.