Here's what I think. I think that, as always, Apple wanted to have sufficient supplies of all models and accessories to cover launch demand. And, as always, they had to juggle a lot of pieces in advance of the launch, including announcing a launch date, and coordinating thousands of supply-chain details.
With the watch, this was all more complicated than usual for reasons we've all heard: new product category, new technologies, new materials, new manufacturing methods, and not least, being new to luxury and fashion markets, where one of their biggest challenges would have been trying to predict how many people would order which of 38 different watch models.
Other complicating factors: committing in 2014 to an "early 2015" launch (which I've heard interpreted to mean "before the end of April"), and at least one serious component manufacturing problem (the Taptic Engine), and probably others.
Evidence suggests there are, or were, manufacturing challenges for both the Link Bracelet and the Modern Buckle. The problem with LB production probably has to do with ramping quantities, likely related to the sheer number of precision machined parts involved. The Modern Buckle problem: maybe a clasp retention design issue they are scrambling to fix.
In any case, it is into this setting that Apple was forced to launch, warning users in advance that supplies would be tight, adjusting in-store availability plans, etc. The delivery windows they've given us are the expression of all this, representing the best and fairest promises that can be made of a very complicated situation. So far they've honored those promises, which I take as a sign that they've got a handle on things now (except maybe as regards the Modern Buckle, which is still completely MIA).
It's petty for us to think that Apple is somehow being unfair, or not doing all that can be done to move the process along. I'm constantly amazed they can do any of it as well as they do. What other company could have done better?