The reason why we didn't do that was that it really doesn't integrate well with our release process - we release ready to use tarballs, while the VCS doesn't contain all things end users expect (eg. byte compiled localization files). Putting generated content to VCS didn't sound right and there is no option of using own tarballs on Packagist repo.
That's why we've ended up providing own channel with release tarballs. However this approach is not good either as that already bundles dependencies installable by composer, possibly causing problems when trying to upgrade these.
Therefore I've decided to generate separate VCS for composer packages. This way it doesn't pollute development VCS, but still Composer gets what it expects. The phpmyadmin/phpmyadmin is now using separate VCS and is updated daily using shell script. There might be some glitches during initial runs, so please report me any problems you see.
You can find more information on installing phpMyAdmin using Composer in our documentation.