Why do some aircraft require type ratings to fly them?
The FAA specifies the airplanes that require a type rating in 14 CFR 61.31:


  Sec. 61.31 - Type rating requirements, additional training, and
  authorization requirements.
  
  (a) Type ratings required. A person who acts as a pilot in command of
  any of the following aircraft must hold a type rating for that aircraft:
  
  (1) Large aircraft (except lighter-than-air).
  (2) Turbojet-powered airplanes.
  (3) Other aircraft specified by the Administrator through aircraft type certificate procedures.


So in short, large aircraft (meaning that they weigh more than 12,500 lbs.), aircraft having turbojet engines, or any airplane deemed suitable complex that it needs it by the FAA need a type rating in order to fly it as PIC.