If you're referring to the September keynote, the generally accepted reason is that, similar to announcing the original iPhone in January 2007, they didn't want it to leak through supply chain or regulatory filings, and there is no risk of cannibalizing sales of any existing product since it's the first of its kind.
If you're referring to March 9, that's a little less clear. Here are some theories I've heard on various podcasts and blogs. Any of them seem equally plausible or implausible...
- Maybe they originally intended to ship in late March or early April and couldn't easily change the booking for Yerba Buena for the keynote
- Maybe they wanted to give review units to fashion publications, particularly print magazines, which most likely have a longer lead time
- Maybe they wanted reviewers to have more time to "live with" the watch rather than just a week or two
- Maybe they feel like an entirely new product category needs longer to marinate in the public consciousness vs. "just an updated iPhone" where everyone already knows what to expect
- Maybe they needed the extra time to renovate the retail stores and train the retail employees, but wanted to get ahead of any leaks that might result from that
- Maybe they felt like the extra time would create more buzz, despite whether that has actually happened or not