Clearly, Sanders' performance in CO/MA/MN/OK/VT wasn't bad at all. It was his crushing losses in the South that made it a bad night for him. Case in point, the biggest mistake of the Sanders campaign was only focusing on states he thought he could win. In a proportional system, it's worth your while to invest in Texas, even if it's not a state you can win. Better to lose by 15% than 35%.
Ironically, the Sanders Campaign mistake in 2016 was the same mistake the Clinton Campaign made in 2008 (although it was the caucus states she ignored in 2008 rather than writing off a region). She learned from last time.