Because the smog lords said so and you should not be in awe or puzzled .What they say goes ,it is their field we just get to take a couple of turds on it once in awhile . In the future they will add 1996 and newer to OIS , so don't be surprised when they do .
I don't know for sure but I think that the VIN is generally readable through OBDII on 2000 and newer plus there are many 96-99 that aren't fully OBDII compliant. Look at a Ford Ranger for example. It has an evap monitor that only checks for purge flow, no leak check.
I've hit rock bottom but I brought a pick and a shovel and I'm ready to dig!