What run4yourlives said.
To put it simply - when all the wealth is equally spread, there is no incentive for anyone to do anything. If everyone is paid equally and has the same benefits, there's no point in working harder or smarter - you just do the bare minimum and be done with it.
Let's take a capitalist factory: workers are paid $5 per assembled iPad. Those who want more money can assemble 100 iPads and make $500 in a day's work. Most are happy with $100 per day, so they'll assemble only 20 iPads (let's say that's the minimum amount required).
Now a communist, state-operated factory: workers are paid $100 per day because everyone is equal. Those who want to make $500... can't, even if they assemble 1000 iPads. So they just assemble the bare minimum of 20 and are done with it, secretly cursing the government for this injustice/inequality (how ironic is that?).
The overall production level drops significantly unless the state raises the minimum amount of iPads made to 100 per day. Obviously, the people who are happy with $100 per day are now pissed off, either because they don't want or simply can't work that hard.
The only ones benefiting from communism are those who make the laws and control the factories/country.