The big reason, from my vantage point, is that socialism removes profit motive. People will simply not work if they will get money/goods regardless.
As we can see in the welfare crowd.
The only way to make people work under a socialist system is to force them to work. There is no free and socialist society.
This is preaching to the choir, but:
Give people the choice of "work or go hungry" and they'll find something they can do that's productive.
Let motivated people keep the excess of their labors to improve their own lifestyle, and you'll see productivity beyond imagining.