Think many Remain supporters have walked into a trap saying circumstances call for a Brexit extension. Even if just meant sensibly, it is far too easy to spin as “look they’re still using any excuse to try to stop Brexit”.
Plus there’s no simple existing mechanism to actually extend it – that passed in summer. So at this stage if there’s enough goodwill etc to extend, there’d presumably be enough to just do a deal.
(I was all in favour of extending the transition period during Corona, back when this was actually practical, which was six months ago. Now it’s a fake solution that actually helps the UK government frame the debate in a way it likes and is familiar with.)
Good thread on why this isn’t the “common sense” suggestion it might look like at first, either: https://twitter.com/georginaewright/status/1341012943698276356