Can't risk the shiny 'no no-deal' law getting promptly overturned, I suppose. Which makes me wonder what's next. Another referendum isn't going to look much better.
Boris will ignore the extension law passed by parliament because he can't alone decide on a delay the EU has to support it.
The only thing the opposition can do is support an election but they won't because they will lose their vulnerable seats to Farage if they run on a remain or delay platform. So parliament will shutdown for a bit until the queen's speech.
Boris will fly to Brussels to get a 'deal' and the EU will kindly tell him to stuff it because he hasn't send anything to Merkel or Macron in terms of new proposals when they gave him the opportunity to do so.
Regardless the Germans and French have together cobbled up a new deal for Boris to take back home. The sum the UK has to pay is lowered a bit and the terms are more favorable, however the backstop is unchanged.
Boris will return with this new deal from Brussels. The house will vote on the brexit deal again.
The deal doesn't pass because of the backstop, Corbyn doesn't want to risk an election and even if he wanted to at that point there is no longer time to arrange anything.
Boris leaves the EU without a deal as soon as the house votes the deal down.