Americans spend a lot of time in their vehicles. Big vehicles are simply more comfortable and practical on a daily basis, at least outside of a few dense cities. If you're going to be stuck in traffic then might as well have some space to stretch out instead of being squeezed into a compact car. Safety is less of a concern for most buyers.

You can argue that this is irrational based on handling or fuel economy or whatever. But for better or worse buyers prefer size and comfort.