The main reason I can see is that Comcast (or cable internet) has no real competition. I don't understand why that is. AT&T, Verizon etc seem to have given up on competing. Verizon stopped their rollout of FTTH a while back nearly entirely, though it has restarted very slowly.
AT&Ts VDSL based UVerse solution is way too sparse, with very long cable runs. Compare this to BT in the UK where most VDSL2 runs they are doing are <500m. They're now looking at GFast to push fibre even closer to customers, getting copper runs down to ~200m.
Ok, so this may be caused by low population density, but I'm not entirely convinced. It must be way cheaper to dig trenches in suburbia USA as many of the places don't even have sidewalks to dig up and then expensively refill, you could just trench along the side of the road.