I could only think of two excuses:
* some powerful entities had made some very big bets in the derivatives market tied to this and it was worth anything to not lose on those trades. Who knows, but seems extreme even for those sorts.
* there is some deep existential reason that UK’s membership was key to the EU enterprise.
The only thing I, as an outsider, could conjecture on the second point was the UK provided “karmic cover” for Germany (and France) to directly control the other EU member Nations.
In the media, Merkel’s statements were seemingly taken as the de facto EU position. Macron made an out-of-band agreement with Merkel, that made no sense if the EU was their platform for interaction. From an outsider perspective it seemed that Germany was running the show.
But remember Napoleon? Remember the Kaiser? Remember Kaiser 2.0 (Hitler)? But then remember the UK...they didn’t try a “Deutschland over all of us” or a Napoleonic union at the point of a bayonet. They comported themselves rather well.
So the EU really isn’t Germany finally winning WWII if the UK...hey wait, UK, where are you going?
That might be worth distorting the public debate in the UK over...
Purely an ignorant outsider’s take.