Scheizer happens, man. We are creatures of habit, especially as pilots. When you have done things for a certain way for several years, some habits are hard to break. I'm not being critical of your statement in the least, but have you ever missed a checklist item, wrote down wrong performance numbers, messed up the load manifest, missed something on preflight, forgot to check in, missing an approach chart, etc...?

Sometimes the wrong circumstances can all come together at the right time and lead to an attempted takeoff with flaps at 0. Getting to the aircraft late from a last minute airplane change, have mx come out to defer an item, hurried getting ATIS and clearance, pushback to deice, then taxi out with a short taxi and all of this happens on the last leg of a 28 hour 4 day trip.

I haven't seen the checklist yet, but they are doing the best they can of breaking that chain of events to prevent an indavertant departure with flaps at 0.