I think the issue here is how well connected such a port would be. From Rotterdam you can go all the way to Germany through a river. Not sure you could do the same from a port in the Mediterranean, as the terrain tends to be mountainous there.
Add to that, the Dutch have been a maritime nation at least since colonial times.
While one of the biggest ports lies at the footsteps of Athens, the economic power of neighbourg countries is not quite as strong.