I think there are a few factors that could be at play here. Perhaps most likely is that Apple simply did not want the device to leak ahead of time. Had they waited until it was closer to launch, there would have undoubtedly been supply chain leaks out of Asia (and possibly even regulatory filing leaks).
These leaks are obviously now beyond rampant with regard to the iPhone, as we seem to know everything about these devices before Apple announces them. I imagine they care less about leaks in this case simply because the product line isn’t new and they care most about launching the iPhones in a quick and orderly fashion after the announcement.
Remember that when Apple first unveiled the iPhone in 2007, they did so six months before it was actually available — even more lead time than we’re likely to get here with the Apple Watch. That also ensured the device was at least somewhat of a surprise.
Unlike with the iPhone, Apple will be allowing for third-party applications from day one with the Apple Watch. So this lead time also gives developers some time to think, plan, and build some apps for when the device does launch.
Another idea being thrown around is that Apple had already promised to announce a new product line this year, so while the Apple Watch may not ship until next year, they wanted to get it out there now to keep their word. I’d put a lot less stock in this notion. Would Wall Street be disappointed if Apple didn’t announce a new product this year? Undoubtedly. But if Apple is really in the business of pleasing Wall Street and not their customers, I’d actually believe they’re in trouble.
Also, in the long run, who really cares when Apple announces the new product? The key is that it’s coming, whether the pay off is now or in six months or in a year. The only thing that will ultimately matter is if the thing is any good. And I suspect Apple took their time making sure it was good.
I think the first two ideas are far more likely.