To me, the answer is clear. Envy. Don't we all want to have enough money so not to worry about food, children, health insurance, etc?
Sure, there are many wealthy people out there who are corrupt, just as there are poor people who scam and cheat through life.
I think it's just easier to target people with tons of money because perhaps we feel they have everything they could possibly want in life - at least in material terms - so it won't hurt them if we throw stones at them.
Or maybe it's just that some of us feel that those with much more money than us must have cheated somehow because if we're just as smart or deserving as they are, then why don't we have lots of money as well?