There's certainly an element of institutional inertia and a lack of a push to a clear alternative, although Lisbon contains some very tentative steps in that direction.  NATO perhaps also allows Europe to keep defence expenditure lower and there would probably be a big push to end  NATO if the US insisted European countries pay the full economic cost of the US presence here.

There is also the soft option card being played.  It is very easy for Ireland to be anti-NATO when there is no prospect of  a Russian invasion or attack on Ireland, and very easy to be anti a strong EU defence policy if that means Ireland doesn't have to pay any part of the costs and the real cost of European security is borne by NATO or the major EU states.

notes from no w here