Killing someone under the influence is not an act which would attract lenience from the law in any developed country. The media, especially in Australia, has not been great at reflecting the incident in all of its subtlety.
Bulgarian justices don't think the conviction was safe. Bulgarian politicians want him kept in jail. For example, somebody is dead, but convictions for manslaughter are different to murder. Lots of convictions for manslaughter are accepted every year worldwide and attract the lenience they deserve. This case has been politicised.