How about because their constituents voted leave and they promised to respect their vote ?
But let's say they vote it down because that's what they do ? What then ? What is their suggestion ?
Yes their constituents voted leave but Labour have their own version of it, they have said a number of times that they want to stay in a customs Union with workers rights preserved, looks to me that such a deal can be negotiated with the EU which will also solve the Irish border which I assumed is not very important for a majority of Brexiters.
My suggestion is simple, if the MPs cannot agree on a deal, then let the people decide, simpler than the easiest deal in history, don't you think?