Because it got mixed up with fascism. If the system would appear naturally within a commune it might be beautiful but in all past examples the system(rules) were enforced from the top. Communism as an ideology requires the natural process of development which includes the disappearance of the central force at the end. But that never happened yet. So the so-called communist systems weren't actually communist.
The underlying problem with pure communism is the same thing that was seen in the Hippy Communes (that were patterned on them.) If nobody owns anything, and everybody gets what they "need" then there is no incentive to put in that extra work. Even in Russia and China, as soon as the concept of private property returned, the output of farms skyrocketed because now people had a reason to put in the extra work.
Well, for the lazy people that are paid as equally as the people that work hard, it's a great life, but communism is completely unfair and wrong. People take advantage of the money that comes when they don't do work. They're like today's congressman. There's nothing good about it. The rulers are doctoral, hurting others and keeping them within a city.
Because it looks good on paper, just like all Socialized programs and ideals. Everyone gets a job, everyone gets medical care, everyone gets a place to live, everyone gets a slice of the pie. However, in reality, there's no incentives to do better, and no way to improve your situation through hard work such as is possible within a capitalist system. In reality what communism is, is a system in which those in power live high on the hog and the misery is distributed equally among the masses. If America continues on the Socialist path that it's on right now, we'll be able to experience all of this for ourselves in just a few short decades or maybe even sooner.