Curiosity
Theres plenty of blame to go around and I dont think the place to start is the Rabbeim or Rebbetzins….
At the end of the day every person has a yetzer hora and everyone has bechira so everyone is responsible for their choices. However what boggles my mind is that we and our Rabbonim TOLERATE stores that sell untzniusdik styles and lengths. The store owners have zero yiras shomayim and I dont know why nobody does anything about it. If the stores werent selling it the girls would have a much harder time finding it and buying it. I believe that would solve a large percentage of the problem.
However and I have said this before, in my opinion Tznius issues are indicative of other problems. A girl who is not covering her knees very likely watches movies and reads magazines which brainwash her into believing that to be happy she has to look “sexy” and glamorous…. Additionally these tv shows, movies, non Jewish music magazines and such also promote general immodesty as well as the idea that a women should try and attract men.
So if you want to solve the problem you need to clean peoples minds from the shmutz and make the untznius clothing unavailable in Jewish neighborhoods.