Threaded is always better for adjusting fit. That is it's only non-aesthetic advantage.

The migration to threadless wasn't just about marketing, or cutting costs. Threadless dominated from the mountain biking very quickly because it was simply better and more robust. I don't think I've had my handlebars whacked out of alignment once since the time my old mountain bike got switched (in the course of getting my first suspension fork) in like '97 or something. I mean, when you're dealing with an expanding bolt vs. 2 bolt clamp you don't have to be an engineer to see which one is going to be more reliable.

Road bikes took longer to transition because there isn't an obviously felt performance improvement with the upgrade. I suspect the lighter weight in such a weight conscious industry was the kicker, especially as it made practical to make steerers out of lighter materials.

Out of curiosity how is it better at adjusting fit? Because you don't need to cut the steerer tube? Did I just answer my own question?

^kinda. you can set the stem in the steerer tube at different lengths as to adjust the hight.

Well the big one is that you can raise and lower the stem easily, without messing about with spacers or what not.

threaded systems adjust on a skewed cartesian grid, where x is the stem length and y is the range of motion offered by the quill length and the steer tube. this makes it pretty easy to play around with your riding position, even well after your bike is 'complete'. threadless gives you the same adjustability, IF you leave yourself enough excess steer tube when you built the thing to move the stem up and down in the spacer stack until you are happy. otherwise, your option is to keep buying/swapping stems of similar but different angles and lengths to try and find the coordinates you otherwise would have reached by adjustment along the steerer axis. in reality, threadless gives you more options, because most threadless stems can be flipped for a different angle, and spacers are available in basically any thickness if you look hard enough.

couple that with a stiffer interface, lighter componentry, wider availability of cartridge-bearing threadless headsets, wider availability of open-face threadless stems, wider compatibility with bar sizes like BMX and 'oversize', and you have a strong case for a mechanically superior system.

having said all that, i still don't think it looks as nice (with exceptions of the cinelli alter, 3t mutant, and thomson elite), and i still have a bicycle with a loose-bearing threaded headset and a nitto stem.