I think we can only speculate what's going on. But I believe Amazon simply does this to push people to use other Amazon sites instead. Now why they might want to do this in this case is beyond me, but in some way money is probably the answer.
Similar things have happened with Amazon.de for me. Amazon has launched a Dutch front (Amazon.nl) and really tries to get Dutch customers to switch from amazon.de to amazon.nl (along with their Prime membership). Besides getting spammed a lot, certain product also ended up not findable through the search. Some products even stopped listing Amazon as a seller, but if you knew Amazon's seller id (or had it in your "save for later" list before that point), you could still access it. At one point, these products could also no longer be "delivered" to the Netherlands once you were at the checkout phase.
Mind you, it didn't report them as "out of stock", but the end result was practically the same. It seemed like a lot of products were no longer on Amazon.de, but they were on Amazon.nl. The real kicker was that the things that I did order through Amazon.nl, still ended up coming from Amazon.de's warehouse after all.
I'm not saying that this is the case here, but I'm also not surprised by these practices. Whatever the reason, pretending an item is out of stock or simply not sold by Amazon likely results in less confusion/backlash than having a text stating
"this item can't be shipped to your country X".