Because communism doesn't work for large, heterogenous groups, so increasing amounts of coercion are used to keep the system running.
And new forms of government such as socialism are generally more succeptible to corruption as people find the new loopholes; as a government gets more corrupt, those who corrupted it seek to consolidate their power.
I think socialism can be made workable, as we examine and correct the problems with previous attempts. I don't think communism works well for human societies, as it requires people to act better than we know they do.