Boredom? That is, if you're outside, you're probably more engaged in your environment, whereas on the treadmill, you're in a static environment, where you can focus on all your little aches and pains.

Otherwise, I'm with Mark's assessment. The same sort of thing holds for free weights, versus a weight machine (depending on the machine, of course). The former requires all of the surrounding stabilizer muscles, while the latter inhibits motion so that they never come into play (or do so in a comparatively reduced fashion).