It's been missing for about a week now. You can only buy it from a couple third party sellers. Amazon still sells the Defender for the s8 though and all the other Otterbox cases for the s8 Plus but not the Defender.
I would honestly just call Amazon, I have called their customer service before and they were actually amazing. I am sure you would get a far better answer by calling than someone here guessing at this point
Maybe they are waiting to the new one with the crappy screen protector as part of the case?
Wish i could get one of those for those "heavier" days =/ unfortunately i'll pass on every future otterbox case as they're extremely expensive in canada (80$ - tx) i can hardly justify paying that kind of money for something i could get cheaper that could do the same
ie: UAG case which if i'm not mistaken i've seen in my local best buy.
Still a nice one to consider if they stay screenless imo since last time i got one i've always had issues with the sensitivity and had to press way too hard imo