We bought a treadmill two years ago to combat some of the Colorado winters. Both my wife and I were trying to get into shape and we had enough cold, windy days that we thought a treadmill would make a nice Christmas present to ourselves.
It was as the week of Christmas we got snowed in for 3 days, we bought the treadmill the first day we got out and then promptly got snowed in again a few times that winter.
This year I've started this streak, primarily on the treadmill, and I've enjoyed it. In fact, of the first month of running, I've done all but 1 day on the treadmill. Even on a recent trip to Indianopolis, when I had great weather and was thinking of running outside, I used the Bally's treadmill instead.
Part of it is time. The treadmill is always here, even late at night and it lets me get the run in and then move on to the rest of my busy life. It's efficient to come downstairs, warm up, run, and then go cool down with the kids or my wife or go on with the rest of my life. Outside I tend to go a bit longer, since I don't have an exact point that's a mile, and I struggle a bit more since we have some big hills here. Either I'm struggling up them, or pounding hard down them.
Part of it is that I like watching TV while running. I can feel like I'm doing two things at once, and get distracted a bit. I used to run around some parks and that was fun, I could just run without worrying about anything.
That was one reason I didn't run in Indy. I was in a business area, lots of concrete and sidewalks, I didn't have time (or a car) to find a park, and so for now, I thought it was more fun to run on the treadmill, watch TV, and let my mind drift. Outside I have to pay attention, especially when I'm going up and down sidewalks.
As I travel more, I'll look for some places to run. Already I know I'll have a car in Florida and I like Orlando, so I'll map out a route there, probably the same thing in Seattle, though that will be hard to do given the weather up there in November.