1. Theyre racist and xenophobic so they dont like migrants
2. They feel discriminated against by urban people so they dont like them either
3. Combine those two and its easy to see how they can revel in the suffering caused by the situation. Politics is about narratives and this one spread like wildfire because dems had no counter narrative. End of the day cities are dominated by democratic politics so its easy for republican narrative makers to point at cities that are failing to deal with a crisis and turn that into a reason to not vote for dems.
4. Everyone loves a good told you so. The problem for democrats with this one was that it was so incredibly visible. Made for very good TV on fox news