Because you can maintain your SEP rating on a VP1, with three steam driven instruments and performance that just about out-climbs a bicycle up a hill, and can't on a CTSW with 120kts cruise, a suite of electronic instruments and a good enough climb rate to give most helicopter pilots a nose bleed.
(And equally a microlight pilot can fly the CTSW with no additional training, but needs a whole extra rating to fly the VP1).
G