Burnham almost had an open goal at the leadership race but by abstaining instead of opposing on the Welfare Bill he allowed Corbyn the opportunity to be seen as the candidate who would oppose the government and that momentum propelled him into Leadership of the Labour Party, a problem with politics is it’s a complex business that is seen as a binary concept by voters.
I think Burnham's one of the only Labour politician that seemed to learn anything from Corbyn's popularity and success in the 2015 leadership election. While most of the rest of the PLP decided to sabotage Burnham's one of the few who actually modified his approach and became much more left leaning.