I keep asking myself the same thing... My explanation is this: in summer, there are other/more fun things to do outside that I cannot do in winter (like rock climbing).
My schedule tends to declutter itself starting november and stays pretty stable and predictable till about march. And since I go running with a colleague of mine, it becomes easier to sync our schedules and go for runs together.
An additional factor, winter seems to affect me pretty badly in Germany. I get stressed faster and my mood suffers from it. Running helps against the stress and mood swings.
edit: AND, running at -10°C next to a partially frozen at dawn is damn majestic.