Comment by Sharlin

The UK already enjoyed a large amount of unique concessions as a full member. The rest of the EU was right not to give in. Also, game theoretically it's of course more important to not make leaving (or threatening to leave) the union seem too lucrative. No such problem in negotiating tighter integration with nonmembers (and in retrospect, many of the old WP countries should've joined as something like associate members at first…)