I agree that flossing is more involved than brushing. I think what tipped the scale in its favour is ease of location. I can grab my floss and floss in my room, sitting on my bed with my legs stretched out whenever I feel like it. Brushing on the other hand means I have to get up, and carry my stuff down 2 floors to get to the bathroom. I know this isn't a really big deal, but I seem to view it as being more inconvenient.

Didn't floss last night.