dddd wrote, "So Peugeot's designers actually wanted the 57cm top tube length on their 54cm frames, sounds long but it is what it is."
My best guess is that they wanted the steep seat tube angle to place the rider farther forward and the steeper head tune and shallow fork rake for quick handling, and they wanted a consistent center-front length, so the long TT was simply a consequence.
The larger question is why they did this for a few years and then reverted more or less to previous geometry.