The curriculum didn't occur in a vacuum, though. It's based on what the leaders thought the students could handle. No one wants a curriculum where a lot of students fail.

Part of that is heterogeneous grouping. The curriculum is based on what the bottom 10% can handle, and everyone else must go at their pace lest there be elitism.

There's probably other factors too, though. All students could probably move faster if culture as a whole were encouraging it.