S3 is HTTPS-based, and all you need to implement it is access to HTTPS and HMAC hashing (for computing signatures). This allows it to be used with virtually any platform and programming language, because uploading a file is as simple as making a HTTP PUT request with a few extra headers set (assuming there isn't already an official or unoffical SDK that you can leverage instead).

S3 is considerably simpler than SFTP or rsync over SSH on non-Unix-like platforms, which makes it a very interoperable API. Even Wordpress or a small Go service can implement S3 support without much effort or any extra dependencies. Being HTTPS-based, it can also be reverse proxied more easily because HTTP presents a Host header, unlike SSH.