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....)

Here's [a google-search images for 'door handle'](https://www.google.ca/search?q=door+handle&hl=en&tbm=isch&tbo=u&source=univ&sa=X&ei=202DU6P3FJCkyATHpYHQCQ&ved=0CEAQsAQ&biw=1536&bih=722).

Note how almost all the handles are specifically shaped.

![enter image description here][1]

Those that are not specifically shaped, often have a base-plate that makes the mounting different for different door-sides.

![enter image description here][2]

Only a few of the handles have a round/ambidextrous handle **and** mounting plate.

![enter image description here][3]

Out of interest, door handles normally have a 'stop' built in to the handle which limits the handle's travel to avoid stressing the actual bolt mechanism in the door (on the other hand, knobs don't have that leverage, so they let the mechanism in the door be the 'stop'). This mechanical stop requires the handle to be securely fastened *to the door*, so handles (with their additional leverage) need a stronger mounting (the mechanical stop *in the handle* also often prevents you from being able to reverse the handle on the mounting plate).

With the ambidextrous mounting plates on some of the handles, you would probably use a stronger door/lock in order to support the leverage on the handle, which transfers the cost to be elsewhere....


  [1]: http://i.stack.imgur.com/sr6os.jpg
  [2]: http://i.stack.imgur.com/mz4s8.jpg
  [3]: http://i.stack.imgur.com/cF6Pc.jpg