eonline.com offers one explanation to why this information is being made known now.

Quote:
"While working on the sixth Potter film, Harry Potter and the Half-Blood Prince, which focuses largely on Dumbledore and Harry's relationship, as well as the elder wizard's interaction with a young Voldemort, Rowling said that she slipped director David Yates an eye-opening note after noticing that there was a reference in the script to a girl in Dumbledore's past."


Dumbledore's history may not be relevant to the story, but I can certainly understand if a character's creator wants to keep true to the character she has created. I bet more than a few people here can relate. :)