Wikipedia suggests this is because English copies Latin usage of placing the abbreviation before the year number.
Since AD is a latin phrase and BC is not, we arrive at 535 BC and AD 1066.
Nevertheless, supposed acronym derivations are the largest source of folk etymologies - those popular fallacies about the origins of words or phrases.
With many of these false derivations the word comes first and then some suitably chirpy phrase is invented to match it.
In general usage, although not in the piece below, these restrictions are frequently ignored.