If you were an American hiding in Sicily, would you have a big wedding and invite the whole town? Perhaps put an announcement in the local paper: "American hiding in Sicily to Marry local girl Appolonia"? Hundreds of people would be asking about the stranger that Appolonia married. Word of mouth would travel to the next towns. Could Michael had broadcast his whereabouts in Corelone any louder? No. Meanwhile, Fredo was well protected on the West Coast. Fredo was smarter that Michael when it came to hiding! It was Michael's own doing that they got to Fabrizio and blew up Appolonia.