Because he is publicly committed to renegotiating the withdrawal agreement.
Which he will fail to do.
A referendum or a general election.
He can try and win by empty popularity, I suppose, but there's more to it than that.
And in Scotland all the remainers are going to go to the SNP, while the Brexit vote splits.
As the only two remain parties, they're going to have a clear run in Scotland and England
It's going to be a lot closer than they think