I would say the main deciding factor for that was the rejection of the Treaty for Establishing a Constitution for Europe (https://en.wikipedia.org/wiki/Treaty_e

I would say the main deciding factor for that was the rejection of the Treaty for Establishing a Constitution for Europe (https://en.wikipedia.org/wiki/Treaty_establishing_a_Constitution_for_Europe https://en.wikipedia.org/wiki/Treaty_establishing_a_Constitu...), which would have given the European Parliament far more power, in plebiscites in Belgium and France, causing other countries to bail on theirs.