When is an aircraft required to be pressurized?
The main reason is that it's simpler than having everyone wear supplemental oxygen masks. There is no specific requirement for pressurizing an airplane but there are requirements for supplemental oxygen as per FAR 91.211 


  (a) General. No person may operate a civil aircraft of U.S. registry--
  
  (1) At cabin pressure altitudes above 12,500 feet (MSL) up to and
  including 14,000 feet (MSL) unless the required minimum flight crew is
  provided with and uses supplemental oxygen for that part of the flight
  at those altitudes that is of more than 30 minutes duration; 
  
  (2) At cabin pressure altitudes above 14,000 feet (MSL) unless the required
  minimum flight crew is provided with and uses supplemental oxygen
  during the entire flight time at those altitudes; and 
  
  (3) At cabin pressure altitude above 15,000 feet (MSL) unless each occupant of > the aircraft is provided with supplemental oxygen.


So by pressurizing the plane to under 15,000 ft you can fly over that altitude and not need to supply the passengers with supplemental oxygen for the duration of the flight. There are unpressurized piston planes that fly well into the flight levels but are required to carry supplemental oxygen and everyone aboard needs to use it, it's a look some people in general aviation don't mind, 

 
(source)

On a slight side note the engine does have an effect on this choice when it comes to light singles. Pressurization systems on turbo props/light jets pull their air from the compression stage of the turbine which is already compressing air so no extra compression hardware is needed. If you have chosen a piston engine for your design you need to drive some form of pressurization system which can add weight and complexity. There are a few pressurized piston planes out there like the Mooney Mustang, Cessna 210 (came in both pressurized and un pressurized) and the Piper M350 (the only one currently in production).