I read about this in the MileHigh newsletter:
"We have heard that #0 was severely under-ordered by fans and comics retailers, alike, and thus will likely end up being the key issue to this entire crossover event. With just an estimated 60%-65% as many of this key prequel issue ordered as the actual first issue to the series, it seems mathematically impossible for everyone who ordered the #1 issue to also obtain a copy of #0."
Wow, that's wild. I ended up getting a friend to grab me a copy in his town later in the day so it's all good. But, yeah, my LCS owner told me he only had X amount of preorders for the issue, then after he ordered it he got X amount more preorders so he had none left over for the shelf.