I can think of a couple reasons:
1. People are stubborn and don't like change. We've all grown up knowing what a mile is, an inch is, a pound is, etc. Changing to metric would feel foreign and strange to many people.
2. People don't care about simplicity. Really, they don't. Again, people don't like change, so familiarity > simplicity in the eyes of most.
And if you think about it, the average person doesn't need metric. If a person understands "inch" "pound" and "mile" but not so much "centimeter" "kilogram" or "kilometer" why should they have to change? To satisfy some politician's dream to be "more modern" or "progressive" or whatever they come up with?
Scientists, should of course use metric. It's basically the standard in science, and for good reason. But for the everyday American use, who cares? Not me.
