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Doorknobs are ambidextrous.... you can install them on either side of the door.

Handles are normally right/left handed, and can only be installed on one side of the door (i.e. you need a left/right pair for each door).

You will likely find that less material is needed for a doorknob as well, so they are cheaper. The handle on a handle provides leverage, which means it takes less weight/force to break the mechanism, so a Handle's mechanism needs to be stronger to support the same forces a doorknob would be exposed to. Also, just the shape of the lever means there is more metal..... A knob would have much less raw-material than a handle.

If you expand the UX scope to include the sticker-shock when you purchase one, then knobs have a better UX ;-) (and, also probably a smaller 'carbon footprint', environmental cost....)