A-Best: Because at the time when the interstate was going to be finished in the 70s, the US was supposed to be going metric. So the state wouldn't have to change the signs to metric later, they just put them up in metric to begin with.
Because it's confusing, they are NOW just starting to change them over to miles. They started at the top and are working down to the border as the state has the money to do so.
Delaware Route 1 was also signed metric when it first opened in the 90s for the same reason. The people hated it and it didn't last long. The only thing left is that the exits are still numbered by the kilometer instead of by the mile.
