I don't think it has anything to do 'them' enjoying 'portraying rich people in a negative light'. That's a bit absurd.

I think it just makes for better drama; the villians are rich because they're accomplished and therefore smart and harder to catch. They also would have access to whatever financial or social resources necessary to help conceal the crime or lead Columbo down a wrong path. Finally the fact that they have so much to lose adds to the drama considerably.

I remember Falk saying something along the lines of
here we have these wealthy, powerful people with the whole world at their fingertips. In a material way they have everything that one could possibly want, they are god's chosen..but it's not enough, they still want more. It's their fatal weakness and in the end their downfall.
Without getting to Karl Marx here I would say the clash between classes is probably the single major reason I love Columbo so much.