A reason for this could be a lack of aerodynamic warning or other issues that can be worked round by giving the pilot a clear warning of an approaching stall.
Whilst I agree that it can be a certification issue and limiting for this reason,there are very few modern training aircraft that are lacking in aerodynamic warning (how many instructors would be unhappy in a modern Piper or Cessna with the warner u/s?). The only aircraft I have flown that was lacking in aerodynamic warning was the Emeraude and that did not have a warner fitted anyway! This does show though that a proper MEL is a good system and takes the guesswork out of it.
