Whatever the merits (or lack) of Brexit, it was put out there as a cynical re-election promise by the Tories in hopes to head off spoilers from UKIP. They didn't expect it to pass and most of them didn't want it to pass. It was mostly sold as something it wasn't, including ridiculous claims that Brexit would allow more money for the NHS, because the EU was apparently taking all the NHS money (not). Then it passed and for months everyone talked about a "soft Brexit" (whatever that meant) then a "good deal for Britain" (whatever that meant) and now it basically has become a fuck you, we're out, and we have no idea how we're going to deal with the thousands of complex issues including what to do about all of the current EU residents of the UK and all of the current UK residents elsewhere in the EU.
Cynicism, folly, and incompetence.