Because the Church is slow to change and rarely very innovative. Saturday afternoon confessions are a holdover from a time when Saturday was entirely different culturally, people had the time, and there was an expectation that they should go to evening confessions (which were emphasized more as a whole) Saturday in preparation to receive communion Sunday (and preferably before going out and getting drunk or rowdy Saturday night).
The timeframe and culture isn’t suited to such a practice any longer. Parishes which have found a way to emphasize the importance of the sacrament and make it convenient for parishioners do quite well. Where no one has enough of a clue to try something different which might be more responsive to the actual times, it gets cut to 15 minutes or a half hour on Saturday and everyone scratches their head asking, “Why doesn’t anyone come to confession?”