Cozumel is best known for diving — the reefs off its southwest coast, part of the same system that runs down to Belize, made the island's reputation. For a wedding, the relevant consequence is the water: the leeward side is protected, exceptionally clear, and calm in a way the mainland beaches around Cancun often are not.
The island is also less built up than the Riviera Maya. There is one town, San Miguel, and a single ring road; away from the resort strip the island is scrub and empty coastline. Couples who find Cancun overwhelming and Tulum too self-consciously styled tend to end up here.