Because 2 years of further wasted time and uncertainty damaging the economy is unacceptable, but sticking to the latest agreed date and maximising the chance of the EU becoming aware that no deal will happen if they do now't and they'd forced it, was ok. Or at least the least awful option. One's not even sure one can legitimately leave before then anyway. Trouble is the EU are split with one part admitting they have no intent to become reasonable and the other part, equally not bothered, telling Johnson he has to come up with something the EU can consider then reject without justification. Roll on Halloween.