The big reason for the switch to threadless is due to weight. Not of the stem, but of the steerer tube. Newer forks switched to aluminum steerer tubes and the threads did not hold up. Someone (who I don't know) decided that a friction clamp would work and not weaken the connection. Thus, threadless was born (or invented). Now some manuf. (like Cannondale) are using Carbon Fiber steerer tubes.
I don't think quill stems will ever dissappear, but threadless will (has) become standard.
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