I'll cast another vote for "weather".

I'm only 5 km from work, so it's pathetic that I don't bike more often. However, Calgary weather is variable at best, and -30 with conditions described as "ice crystals" at worst. My beater bike apparently won't brake when it's the slightest bit wet, and its 5-speed gearing makes the one hill between my house and work just that much more of a deterrent.

On the plus side, I will often run in conditions that I don't want to bike in, so it's probably good to force me to cross-train!