Each side of each trade has to come out ahead, which can only happen if they disagree.
A one-size-fits-all program, on the other hand, leaves half the people ahead and half the people behind.
The people left behind aren't given the option to decline the trade.
The standard of living of the nation does not rise.
It's automatically worse than a free market, even if it's a good program.
A typically bad program is a disaster.
Socialism fails because it can't convert disagreement into a rise of the wealth of the nation, even benevolent and competent socialism, if that were a stable possibility.
Bureaucratic incentives always make bureaucratic benevolence unstable.