What let most European nation-states to give up parts of the sovereignty and unite in steps from the ECSC to the EEC,

What do you think let most European nation-states to give up parts of the sovereignty and unite in steps from the ECSC to the EEC, EC, and, finally, the EU? And, conversely, why have some states such as Switzerland or Norway opted to stay out of that process, or at least just be bystanders cooperating in the European market?