From the video I honestly don't believe a 'weight cut' was possible for welterweight. He would have needed to diet over the past month to get into range, then water loaded by the time he headed to Vegas.
His body fat is too high to for a water cut from where he is at.
So either Nick was sick, injured, or otherwise unable to diet and do roadwork, or he lost track of how calories work and the weight wasn't off in time. If its the second option, its really unfair to Robbie, which would be disappointing as a big Diaz fan.