What's the difference in threadless and threaded headsets and stems? Why change it? is it because it's easier for maintenance?
There's a page on here explaining the basics, Terminology index - a list of bike part names and cycling concepts
There are some advantages for maintenance, it is simpler and faster to adjust the bearing preload with the central bolt through the top cap, rather than using the lockrings on a threaded headset. Threadless also requires only hex keys to adjust, as opposed to specific lockring spanners.
The overall system of threadless headset and clamp on stem is probably lighter and stiffer, but I'm guessing... if true it's a big reason! i.e. lighter weight and better steering.
The larger stem with threadless headset allows material for the modern handlebar clamp with four bolts, enabling you to remove the whole handlebar assembly easily. It is able to achieve this without significant weight penalty. With a lot of quill stem designs, the clamp forces you to remove brake levers etc from the bar to remove the bar from the stem by sliding it through. There are two bolt clamp designs for quill stems but arguably 4 bolt clamps are a superior design.
With threaded, the quill stems have a tendency to get rusted in place, worsened by aluminium vs steel combinations, so rust or galvanic corrosion can prevent removal of the stem, thus adjustment or bearing servicing becomes impossible if a threaded headset is neglected for too long. This is less likely to be a problem with a threadless headset.
Also for threaded steerers, the steerer must be cut to fit the frame head tube and the headset stack height quite accurately. Replacement headsets must match this stack height quite closely to be compatible. Whilst this is an unusual occurrence, more significantly the fork/steerer tube cannot be re-used in a taller head tube on a different bike. Both these issues are more easily managed for a threadless system.
Hope that helps, no doubt there's more reasons than this!
As to the what, others have pointed to other resources.
As to the why.. From all I read it appears to be because of MTBs and the aftermarket fork sales. The threadless fork is easier to stock since it can be fewer sizes, it can be cut to fit the bike. Whereas the threaded fork has to be cut to the correct size, or rather manufactured to that size. Thus this makes it cheaper to stock the threadless forks. It doesnt sound like the right reason :D But from all I read, this was the primary reason. There are other reasons that others have pointed out, weight, stiffness etc. I dont like the reason that the larger stem makes it easier to remove the handlebar easier because I feel they could have done that with quills as well.. but that is an advantage for most people.
Another advantage is it is easier to adjust or assemble/disassemble. I spent weeks/months tracking down the right tool to tighten my threaded headset, but threadless are typically fastened with very common hex key size screws.
But I am not convinced of the original reason either because all the cheaper bikes have threaded stems :D. Although it might be that standard designs such as MTB and road are easier to use with the threadless design, whereas the threaded system allows for more variety such as my foldie.
I was curious about this.. because I find the whole adjust the height with spacers concept very hacky. Spacers are typically used in other places to hack stuff that dont otherwise fit after all..