For team GB, my guess about the situation is that plan A was to ride for Froome and there was no plan B. So Cavendish and Rowe do their bit, pulling to Firenze, then when the laps start it becomes pretty apparent that Froome doesn't have the form to do anything. Which seemed pretty likely anyway, from his recent results, and probably the GB riders had the real skinny and started the race knowing theirs was a lost cause, or figured it out pretty quick - they know each other well, can read the body language. Now do you suffer 270 kms to support a leader who will be lucky to just finish? The rain and slick pavement and falling riders has to enter into your thinking. A fall means some risk of injury to the body that you depend on to earn a living, that needs to start next season strong. If there's no possible reward, will you take the risk?

As far as I can tell, in pro cycling there isn't an imperative to finish a race that has gone all bad. The riders are pretty pragmatic about it.