In 18-27-700.6 (b) "Emergency lighting systems shall be designed and installed so that the failure of any individual lighting element, such as the burning out of a light bulb, cannot leave in total darkness any space that requires emergency illumination."
Typical emergency lights are the frog eye type battery backup that are either wall or cieling mounted. I would think this is the reason. With a flourescent fixture, the battery backup is typically either the inside 2 or outside 2 lamps. If one of these lamps burns out, you will be left in total darkness.