Because slamming as moochers the poor and the elderly is the same as slamming the racists, homophobes and anti-semites who are Trump's base.
Yeah, really. Romney seemed to cement himself as a heartless plutocrat with that comment, but Clinton here is just calling people out for what seems to be obvious, at least if you go by people at his rallies and polls showing the absurd beliefs of a sizable number of Trump supporters.
Plus, going by the numbers, she only really insulted like 20% of Americans, which is probably lowballing it in terms of # of sexists/racists/xenophobes/etc, whereas Romney basically said 47% of Americans were poor moochers that he didn't really care about appealing to.
I don't hold that comment against her at all. I'm actually slightly glad a politician didn't attempt to gloss over the fact that a large portion of our country are indeed bigoted assholes. I sure as hell am not making excuses for those people.