Most states have reciprocity agreements that allow out of state drivers whose cars are legal in their home states to drive legally in the foreign state.
Meaning, as long as my car meets Mass regulations, I can legally drive to PDXFest '10 and back, even if my Mass legal car violates NY, PA, OH, IN, IL, IA, NE, DE, WY, UT, CA, OR, WA, OR, ID, WY, SD, and MN laws.
Similarly, many states issue insurance cards. Mass does not. So if I get pulled over in a state that issues/requires insurance cards I can't be fined/go to jail for not having an insurance card. It's asking a bit much for residents of all 50 states to make sure they're in compliance with all the 49 other states' regulations.
I'm inclined to say it's part of the Interstate Commerce code, but I'm not a lawyer so don't quote me.