Right now it seems that the library Phabricator uses for S3 is quite outdated and it doesn't support the Amazon V4 authentication scheme (http://docs.aws.amazon.com/AmazonS3/latest/API/sig-v4-authenticating-requests.html). This is sadly the only available authentication scheme for the EU-Central region and since we've deployed Phabricator there (for many reasons), we cannot really use S3 as the backing file store right now.
There's also the risk of AWS deprecating V2 before long since V4 has been out for quite a while by now. :)