Very likely because the aircraft is cleared for single pilot operations. I do believe the new system does away with the ALTP requirement for over 5700kg, however, if the flight manual states a minimum of a 2 pilot crew then the P1 requires an ALTP whatever the weight of the aircraft.
If the company requires a two pilot operation then the P1 can be CPL rated in any aircraft cleared for single pilot ops as per the Aircraft Flight Manual.
Have not flown the B1900 myself but have read reports of it being operated single pilot on freight ops.
Prospector