Another reason that they will oppose Labour’s soft Brexit is that they do not want Labour to be able to do something the Conservatives were unable to do.
There are plenty of Labour MPs, backed up by many more Labour party members, who do not want to see Brexit at all, and they might vote against any agreement.
Labour have now said they would hold a referendum on any deal they negotiated, and they would not be allowed to backtrack on this because a combination of Tories (yes, I know, but see above), smaller parties and Labour rebels would insist it be held.
The Tories in an election will want to pin the blame for their failure to achieve Brexit on Labour, and if Labour switched to being a Remain party just before the election that tactic will probably be successful.