It's the cost of change, and also the fact that the people you need to convince to switch don't benefit. If you're an adult in the US, you already know the conversions you need to do everyday tasks (e.g. 12 inches to a foot, etc). You also have calculators absolutely everywhere to do the math for you, so it isn't like you benefit from being able to do it in your head more easily. So, if we switched to metric most of the country would have to incur significant costs, and reap no benefits. That's never going to fly.