Brexit
Brexit would very likely lead to breaking of UK, Scotland preferring EU as independent and Northern Ireland preferring EU as member of Ireland. Tory leadership (unlike Tory and UKIP voters) most likely considers EU membership as lesser evil that UK breaking up and only England and Wales remaining. This, most likely, is the decisive, fundamental calculation.
Next, tactical question for Tory leadership, how to avoid Brexit and put the political blame of remaining on the opposition? As it seems, the tactic seems to be to botch up the "deal" on purpose, and leave it to majority of House of Commons to take initiative to last minute revoke of article 50 nominally "against" the May government, to avoid threat of no deal Brexit, but de facto doing exactly as planned. And majority of the public opinion wrath goes against Labour.