threaded and threadless fork origin
The threaded design is good enough. Sure it is heavier than necessary, flexy, has the single point of failure, the stem can seize in head tube, it is more expensive to manufacture and requires special wrenches to adjust, but none of these problems is exactly a showstopper. On the other hand, it has the benefit that the height can be easily adjusted until the stem is stuck. 

For a long time there was no strong incentive to invent a different system, but when threadless headsets were invented, they quickly took over the market.