I suspect the simplest explanation is that poor people are pitiable in the abstract but disliked in the concrete. People in rich neighborhoods support welfare because it makes them seem more compassionate than their neighbors; people who deliver pizzas quickly learn to dread the days welfare checks arrive.

I suspect we give different predictions for support for welfare in a place where, say, 80% of the population is on welfare. The signalling theory predicts that at most 20% of the population will be opposed to welfare; my suggestion could explain that >20% opposition to welfare.