Because it does not work, period.
If you don't have incentive (you get paid the same no matter what, and don't pay the bills) you don't care. Some might, but most won't.
They did studies showing how the farmers would do a really good job with the outside of the field so that the government officials could drive around to make sure that they were doing well, but the inside would not be tilled/planted so yeilds would suck.
Since here a farmer will not make money if they don't do their best planting to get the max off of it they always do the best they can.
Also since people did not pay their electricity bills (government did) then they would have no reason to shut off lights in factories and would just leave using an insane amount of electricity for nothing.
Another interesting thing is that their allocation of goods was poorly developed so some communities would have a bunch of surplus, while others would have nothing. And they were not allowed to just send it because then the government would get pissed (and in the USSR that usually meant death or prison).
They would have to get permission to paint different colors on the equipment, so if a company that made tractors had buckets of green paint, but it had to be red and they ran out they would have to wait for approval from the government before they could paint it green, meaning that the people that needed the tractors would have to wait sometimes years to get them.
Basically a nightmare. Which leads to corruption (if I pay you to look the other way so I can take more/better kind of thing). Imagine every purchase you make in a year, toothpaste, brushes, gas, every single food item, on and on, the government would have to price it. Insanity.