Summer take-offs in DEN can be a problem: an airplane needs a longer runway -- or less weight -- compared to an airport that's at a lower altitude or lower temperature. And it was hot yesterday in Denver. The same plane with same weight might well have been able to safely take off from an equally-long runway at, say, SFO or BOS.
It's possible that the pilots had decided that they needed to reach a certain speed at a specific point during their take-off roll, and that they hadn't attained that speed when they reached that runway checkpoint.