Dirty deeds and they're done dirt cheap

We've been talking a lot about social class in the past few weeks and the fact people are apprehensive about saying they're upper class or lower class. In our class discussion an idea came into my head. Why are extremely rich people called "filthy" rich and extremely poor people are called "dirt" poor, but middle class people aren't referred to as anything "dirty". There's no "muddy" middle class and I was thinking, well... Why? So I did some google-ing on the origins of the phrase "filthy rich" and here's what I got.

This was first used as a noun phrase meaning "rich people; who have become so by dishonourable means" It has become to mean "extremely rich" rather than "dishonourably rich", although there may still be a trace of an unfavourable implication associated with it.

And as I continued my google-ing I couldn't find a solid definition of "dirt poor" but some of the guesses were that people who were "dirt poor" actually had to sleep on dirt floors because they couldn't afford a bed. I think this shows how the middle class is still idolized in America because it isn't partnered with any name that is unclean. Why is this?