The case for Costa Rica is landscape variety in a small country. You can marry on a cliff above the Pacific in the morning, hold the rehearsal dinner beside an active volcano the next evening, and send guests home by way of a cloud-forest zipline. Distances are short enough that a multi-day wedding weekend feels like a tour of the country rather than a series of long transfers.
The country is also politically stable, used to tourists, and fluent in weddings. Planners, photographers, florists and officiants all work with English-speaking couples routinely, and most resorts have in-house coordinators who have run hundreds of ceremonies. The currency is the Costa Rican colón, but U.S. dollars are accepted almost everywhere, which makes budgeting simpler.
Couples pick Costa Rica over Mexico or the Caribbean when they want something wilder: vows with howler monkeys in earshot, black sand instead of white, a welcome dinner beside a thermal pool.