There are several causes:
1- the ICE has to run more to keep the engine and catalytic warm
2- There is more rolling friction at low temperature (affects all cars)
3- Wet weather causes additional rolling resistance.

Yes cold weather affects the Prius as well. My 2004 has 43k miles and averages 46+ but gets about 40 in the winter (CT) and better than 50 in the summer.