The sole purpose of Brexit, from a political point of view, at every stage, is all about getting the Conservative party out of a hole of their own making, and surviving to the next day, usually by treating it as a PR exercise above all else.
If they’d always been intent on a No Deal, and told the EU27 that, then the EU27 would have just stopped negotiating as soon as they heard - making it clear where the fault lies.
By stringing them along it allows them to paint the EU as the people behind the failure to secure a deal, it placates the non-ERG MPs in their own party and the less strident parts of the Tory commentariat.