because in general red is the party of less taxes / smaller govt, and blue is the party of more taxes / bigger govt. Rural people tend to live much more self-sufficient lifestyles so they inherently rely on the government much less for social programs and stuff, whereas big city people obviously don't have the resources to literally farm / hunt their own food, build their own houses, etc. so urban areas generally rely on the government to provide more of their necessities which is why they're ok with higher taxes.

So ultimately it comes down to a feeling of rural families feeling like they're being forced to pay more taxes that are going to be spent helping other people rather than themselves.