The reason I've always heard is pure cost to convert. All the signs, all the cars, everything that uses distances (all the laws would have to be 'amended' to the new units), etc, etc. There is a LOT of infrastructure in the US. It's a physically large country. Even 'phasing' it in is a HUGE undertaking and would take years if not decades to complete.
I am 100% in support of us converting, but I'm glad I don't have to be one of the people that's going to coordinate the madhouse it's going to be when it's finally decided upon and started.