I kinda get it. It's a bit of that gossip mentality that existed since before the invention of the Sicilian grandmother. And since our Hollywood starlet weddings don't get the coverage, these royal wedding things become the once-in-a-generation schlick-fests for princess wannabes in the same way sports fans have their World Cups. They're public, they're open, they're lavish, and they're completely vapid. Stuff a lot of people love.
If people can watch The Bachelor on TV, they sure as hell are gonna watch this crap.