Short production order meant they could keep it a secret. How many leaks were there of this device? Besides some rumors there were no images, no production image leaks, none spotted during transport, etc. You can't keep a secret if you order 5 million to made and shipped internationally.
No, the more logical explanation is (1) They wanted to keep this on the downlow (2) They wanted to gauge interest (3) Wanted early feedback to make changes later on, if needed
You are all also missing the biggest issue: The Band is not some premier device that is meant to take on the health/smart industry. It is a demonstration device for Microsoft's technology and Health platform.
Why else are they licensing the hardware/sensors and software out to anyone who wants it? This Band is the equivalent of a Microsoft Surface. It is meant to set the standard so that their OEM partners can go out and make their own version of it.
Microsoft is not here to crush Jawbone and Fitibt, they want them to continue to make their devices but with Microsoft integration. By having a limited supply and little advertising, they can walk the fine line of getting this device out there and not completely pissing off the other OEMs. This is meant to light a fire under their ***, not destroy them.
I can say with authority that all the other health/fitness band OEMs were briefed within the last week about the Band and Microsoft's plans. It was an olive branch, not a declaration of war.
So no, no marketing conspiracy, just a shrewd business move.