Great Labour party victory in Oldham

Delighted of course that Debbie Abrahams won the by election in Oldham East with an increased majority. The number of votes received by Labour were higher than what we got in 1997 when we achieved a landslide. The only thing that didn't make sense was why the Lib Democrat vote did not collapse? Instead they came second whereas the Tories were wiped out. So great news for Labour but interesting as to why the expected devastation of the Lib Democrats did not materialise?