I turned over a few different reasons.
I don't think its about supply. I think Apple can easily satisfy the demand. This will sell much less than the iPhone 6. Apple doesnt even have problems with the iPad supply. I think the apple watch will be well supplied. Of course, people are going to think this means low demand. Unless Apple wanted to announce the Watch before the supply chain started leaking it. Oh wait, there already was an event before. They're not getting in front of any supply constrains here
There still working on the OS. Which is obvious. They'll be working on it from now till release. But it might be that because it's a brand new product, so they chose to give it more time. That doesn't make sense. I'm wondering why the event was so far ahead of the release. They could have planned a later event and worked as much on the OS and release it on the 24th either way.
Time to educate employees. I think this is the big one. This would push the event up because inevitably the new info would leak from employee training. Your not going to keep that leak plugged. So you need to get the information out before it all leaks on someone else's terms. Employee training takes a lot of time for a new product, but hardly any for the fourth iteration. This is what the event was getting in front of in my opinion.