I'm not personally interested in it, but I can understand why people are. It's a moment that allows us to intersect with history. As much as people think the monarchy is irrelevant and outdated, it's still a source of influence and a significant symbol. Like watching a space shuttle launch, or shaking hands with a celebrity, or eating at a restaurant featured in a movie, it's a way for people to feel connected with something larger than themselves. There may not be any practical relevance, but the "point" of it all is defined differently for each of us. Some people like to be able to say "I was there," even if only through the filter of television.
On the supply side, the media are more than happy to milk an opportunity like this because it sells a lot of newspapers and helps fill airtime for the news and lifestyle networks. If you're the content manager for one of these big outlets and are given a choice between giving the story the moderate coverage you think it merits or launching a full on two-week blitz, which are you going to choose? Even if you wanted to take a more restrained approach, your competition is busy churning out interviews with the couple's old classmates and experts in British etiquette, with a bit of nostalgia for Princess Diana thrown in for good measure.
Ergo, Royal Wedding Week!