Why doesn't the United Kingdom rejoin the European Union?
Admittedly, I don't know much about Brexit, but from what I have been exposed to, it seems like a decisively economical and political impairment that made travel and business with the rest of Europe more difficult and costly. Since it is so highly criticized as a terrible move, why doesn't the UK just rejoin the EU?
Another factor is that the EU doesn't necesarily want to take the UK back.
The Brexit, and other anti-EU movement in Europe do not come out of nowhere, there is a lot of issue with how the EU works, and some fundamental disagreement between members (and/or political parties within member states) on how should the Europe evolve (Just a big free-trade area, or a continent sized nation with a real political power and geopolitical weight), this is the context behind Macron talking about a Multi speed Europe
A big question is whether we should take new members right now (including Ukraine/Turkey/UK) or deal on the institutional issue and agree in which direction we want to move together rather than taking more people
I agree that the EU needs quite some work to get their acts together. They will basically need a Maastricht II Accord that really turns the EU into an entity that is closer to a national state before they should admit new members. But I can see that admitting Ukraine is something that will probably happen within a very, very short time once there is peace.