Because, contrary to their inferred position during the Brexit negotiations, they need our trade, probably more, or at least as much, as we need theirs.
It suited them to take a 'take it or leave it' attitude when they wanted to hammer the best deal for themselves out of the negotiations, now that they have no power, real or imagined, they have to take a different approach.
They will need to offer far more now, and be more willing to be conciliatory that before because Britain can threaten to walk away, and mean it.
Europe absolutely cannot afford for that to happen - their approach will change because it has to, they will come round, because they need us.