I may be wrong but I think it's because there is no filament in those bulbs. They run on gasses like the long white tube type bulbs. these bulbs need be run at full power to burn the gasses.
Out the front of our house there is an outside light with a daylight detector to turn it on at dusk.
I have just put a low energy bulb in and it came on OK when I switched on. However after a few minutes it started flickering badly so that the light strength fell badly. Seems you can't use it then.