I still don't understand why, when content is digital and distribution doesn't cost anything extra, things like this are restricted to specific countries.
That's because they made deals with content providers to sell you ebooks and audiobooks outside the US, and presumably they don't have those deals for Kindle Unlimited yet. Just because Amazon has the publisher's permission to sell you an ebook doesn't mean that they automatically have the publisher's permission to give that ebook to Kindle Unlimited subscribers.
"Please visit us again when it is available in your country." makes it sound like they are motivated to rewrite them. But you can't just rewrite everything at once. I understand the frustration that people outside the US feel when a new service is launched as US-only first but you have to start somewhere. And a US-based company is most likely to start at home and then branch out.
Content licensing. Different companies (or legally distinct subsidiaries) control licensing & distribution for different countries. Inherent limits of physical books (i.e.: space & mass & energy needed to haul content across significant distances) mitigated the issue, as it's usually cheaper to get a book locally than ship it from afar. Now that content can, for most practical purposes, be shipped free with instant delivery the legal ownership rights are being enforced and controlled (and more middlemen squeezing into). Amazon will get around to sorting out licensing issues elsewhere, but 'til then they've got a big local market to satiate.
I am really sorry people downvote you. People please don't downvote for mere disagreement; not upvoting is enough. You are actively discouraging otherwise completely legitimate and sensible questions.