In short, it is a better engineering solution for the steering system. Aesthetics are a different issue, but if you are tslking functionality the threadless system is a natural evolution:

-Lighter
-Quill stems tend to seize in steerer tubes if not greased regularly. No seizing issues with threadless
-Stem position and headset bearing tension adjustments are quicker and only require an allen key. No headset wrenches
-Locknut loosens commonly on threaded headsets, whereas the star nut and bolt that hold bearing tension on a threadless system don't generally come out of adjustment
-In a threaded system, fork and head tube and headset lengths and stack height dimensions must all match to fit, making fork or headset swaps difficult sometimes. A threadless fork steerer just needs to be long enough.
-No fork threads to get damaged on a threadless system