As discussed in Taanis 31a, it was on Tu B'Av that the men of Yisrael sent their daughters to dance in the vineyard as a loophole to their vow not to marry their daughters off to Shevet Binyamin, in order to keep the Shevet from dying out. On a related note, Taanis 26b discusses that such a minhag continued on afterwards.
Several questions on here discuss the history of this minhag, when it stopped, and whether some communities continued it. But how did the minhag begin in the first place? If you want to say that the case of Pilegesh B'Giv'ah was a case of eis la'asos laHashem, fine, but how wasn't it a violation of tznius for years to come? After all, although one is obligated to see his future wife before marrying her (Kiddushin 41a), one is also not supposed to be out in the public spotlight, looking for attention (Michah 6:8, identified by Makkos 24a as one of the fundamental principles of the Torah).