threaded and threadless fork origin I was wondering why for a relatively long time the most common fork set-up was the threaded + quill stem one. 
Why? 


To me, it makes sense to try to have one point of failure for the the steering system of a bike (handlebar and fork), but it does not seem appropriate that said point of failure is hidden, requiring the disassembling of parts of the bike to be checked for its conditions.
From the fork manufacturing point of view, threaded and threadless fork still share the design of the 2 tubes down and 1 tube coming up, with the possible failure at the connection between the two parts, so I see no reason for making the top part "shorter" and then having a stem reaching down into it to connect.


This question states that threaded stem are stronger, but the "test"[a] does not convince to me as being reliable.

I reserve myself to put here the answer, if I find something useful in the next weeks/months.

[a] holding the front wheel between your feet and checking if you can turn the handlebar without turning the fork ...
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.