If capitalism didn't strangle communism every time people tried to make it work, it might have worked.
True, the Cold War did not help. But you also still have the internal problems of
political purges (which tend select for people who try to solve problems with more purges, and are extremely paranoid about anybody else even getting close to positions of political power - and those who follow them unquestioningly),
rejection of the entire previous system, including the parts that worked (both the methods and the people who got wealthly making them work),
nepotism and the resulting mismanagement (which is also a problem in capitalism but usually self-corrects faster unless the rot has reached the top, because the nepotists cannot retaliate to the same degree with political accusations),
both made worse by
centralization (which requires better administrators, which the two previous points work against),
the resulting
theft, smuggling and backroom deals just to get by, resulting in
organized crime (that sometimes gets co-opted by the state),
starvation if even that does not work (still because of the above reasons),
the
social demotivation as a result of constant political purges (everyone could be a snitch, even family - and if one snitch reports you, all that didn't report you (including family) also become suspects - so the winning move is to not trust anybody, have dirt on everybody (or at least a plausible enough false accusation - the police usually has quotas of how many political dissidents they need to catch, and if they catch fewer they become suspects)
and the
economic demotivation (you do not get paid more for working more, and even risk political accusations from your superiors and collegues for making them look bad in comparison, not much of an increase in pay in higher-qualified jobs (add to that the focus on politics and rejection of the previous system having an impact on who even gets to study), and often there is not much you could even buy with the money)
