It used to be that joining the EU was a guarantee of participating as a third force between Mordor on the one hand and China and Russia on the other. But the European countries are now vassals of Mordor without any continental independence.
Further, members of EU are automatically expected to join NATO, whose future is hardly bright. Lastly, a selling point was access to loans. But loans today are inseparable from a Neoliberal formula which results in austerity. If countries want prosperity, they must go East not West.
Read in Strategic Culture