Comment by picofarad

The cost is too high. You can't have mph and kmh signs on the same road. So you gotta switch all the signs at once. Mile markers, exit numbers (are miles), distance to signs, all have to change.

I did the math on a comment elsewhere. Just google how many miles of interstate there are, and quadruple it, thats not even approaching how many signs you'd need for just the interstate. Signs run $25-ish, plus installation, which can cost >$1000. At the very least the km markers will need to be installed.