> Surely the reason for the vote is obvious. But if I need point it out, it's because remainer MPs (which most seem to be) are trying their utmost to thwart Brexit, against the referendum decision.
No, they are trying to thwart a no deal Brexit.
The simple problem that the Commons, and the country, has is that there is not a majority for anything. Perhaps the largest minority (say, 48%) is for No Brexit. To arrogate every single Brexit vote to one version of Brexit, in particular No Deal which is the most damaging version and the one furthest away from the 48% Remainers, would be both political suicide and undemocratic because it is far from what most people want. There cannot be a "default" option, there must be an option that is positively agreed on. Otherwise, there's a fair argument that the default option is remain, because we couldn't agree how to leave.