Problem is that the CFL electronic ballast (the plastic lump between the bulb itself and the screw in base) doesn't like heat. An enclosed fixture will cook the electronic ballast, and the bulb dies an early death. It won't (usually!) cause a fire or such, but people get unhappy with the bulb not lasting. The fixture will not get as hot as with an incandescent bulb, but incandescent bulbs didn't mind getting hot.
