It comes from the fact that maybe .2% of people that talk about Communism, and this includes those for it, have read Capital, so it's easy to throw around the phrase as a catch all insult for the parts of their understanding of Marx that sound too good to be true. It's ironic because a large part of Marx is suggesting this must happen because of Human Nature, and it's actually his strong appeal to some determinism of Human Nature that is so obviously the most problematic aspect of his general theory.
Marx's issue isn't that he ignored human nature, it's that he placed far too much stock in a specific conception of it.
Maybe im stupid but one of the problems for me with "human nature" is that people mean that "humans like to own shit" but at least in economics people do not like to own shit, people like to enjoy shit (and they do it infinitely) but since resources are scarce they have to own them. Nobody in the right mind would try to own air since is an apparent infinite resource (yeah yeah i know china and clen air).
I can't say I disagree with either of you, but my concern is that there is a more insidious implication to the thoughtless parroting.
By claiming Communism fails because of 'Human Nature', it is implying Capitalism succeeds because of it. I don't think it is a large leap to suggest that this line of thinking tacitly claims Capitalism as more than an economic model; that it is intrinsic to being human (over and above other such models).
Which is creepy as fuck, when you think about it.