The heimish stores in Boro Park (which have gotten less tznius also, although they are still better than the stores in Flatbush) are exactly that- Heimish and Boro parky. and most girls in Flatbush dont like to dress like that. Of course there are exceptions, but most girls want to dress stylish and tzniusdik and theres nothing wrong with that. (Not to say that Boro Parkers are not stylish- they just have a different style than most litvish people)
And besides- it costs A LOT more.