Because the alternative is that, having been through this same-shortages-and-shipment delay thing EVERY SINGLE TIME, Apple still can't use it's huge logistical brains and giant cash hoard to better balance initial supply & demand. EVERY SINGLE TIME.
So while one can read haters suggesting Apple is deviantly holding back watches in a warehouse somewhere to fake demand, another could read Apple lovers flipping what can look like a weakness or failure to better estimate demand into a shrewd marketing play with logical underpinnings: Apple is too dumb to better estimate initial demand such they they are sold out in minutes vs. Apple is so smart to flex it's big marketing brains to leverage the scarcity tactic (which, by the way, doesn't have to involve holding back watches that exist at all, but can also be accomplished by making too few, launching to too many markets at the same time, launching too soon rather than buying a little more time to build extra supply, etc).
If it's as you imply, is Apple dumb for not being able to EVER guess well at initial demand? Of course not. They're smart people. So how does this still happen EVERY SINGLE TIME?
The only way that is compatible with the concept that the scarcity play is not utilized is that real demand always so far outstrips supply that no amount of Apple planning nor no amount of cash spent on manufacturing capacity can ever lead to the availability of more than a few minutes of available supply.
With this Watch, I can somewhat buy that it is hard to make even a modestly accurate demand estimate for a brand new product & product line launch. However, it's the same sequence of events with every iterative (iPhone and iPad) launch too. EVERY SINGLE TIME.
There's no Apple put-down in all that- just answering a post to offer another view of the scarcity marketing tactic in a way that makes Apple look smart, instead of seeming to always lack the ability STILL (after all these years) to better forecast demand such that product delivery is delayed within minutes of going on sale.