User blog comment:FairyRaven/Fairy Tail Chapter 465: 400 Years, Review and Commentary/@comment-27281341-20151207194339/@comment-9595671-20151207195956

It also explains why dragons were something no one believed in, because technically they went extinct and the survivors were in kids not from the current timeline.

Basically, only Acnologia was left, and no one really saw him till the Tenrou incident.