I find that I can run much longer on a treadmill because the pace is set for me. I can't manage it as well myself when running outside and burn myself out too quickly.
Think about the physics of running on a treadmill. The only part of your body constantly accelerating are your legs - your upper body hardly moves. When running outside, you have air resistance, speeding up / slowing down to avoid obstacles, slight side-movements .... Your body simply is working harder.
Everyone can run much longer on a treadmill though. That's not unique to you. I always wondered just how much of a marketing ploy that was - "I love my Acme Fitrun machine - I can do 12 miles in just under 1:30!" Meanwhile that person finishes a half marathon in 3:14.
It's true that running on a treadmill is easier, mainly due to the lack of wind resistance (which is a bigger factor for running than you would think), but also due to the artificially perfectly flat surface; you have to make far fewer micro-adjustments to the terrain when you're on a treadmill, and like everything in distance running, those adjustments add up over time.
It's said that putting the treadmill at a 1-2 degree incline can correct for this discrepancy, and there's actually been a few scientific studies on this as well (e.g. http://www.ncbi.nlm.nih.gov/pubmed/8887211).