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.
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).