Was is that the communist leaders suddenly forgot all social duties and grabbed the wild chances of the new order? Or were the socialist economies so run down (either with austerity or profligation) that there was no alternative but to pull the utopia plug out? (Or was it a self-interested welcome of Western ivestors?)

All of it...
Communist leaders were just a people...if they had some real intentions for social justice ever they would quickly forget it when they would come to power and they became opportunists (take every opportunity to grab money and power). People had "free"(percentage taken from their salaries) education and health system...what else they could need?
Socialist economies were run down for many reasons...I wouldn't know where to start to explain.
Self interested welcome of western investors ... yes. Those in power are now not there for life...their political life span is shortened (all tho not that much)...so now is a time and opportunity and they can't wait for others to sell cheap...soon there will be nothing for sale...