Construction wise, a vari-focus would have effectively an internal tele-converter that moves to change focal length. This adds only two or three elements to the main lens, and without the attempt to automatically maintain focus, you'll refocus and (presumably) hit critical again when you do. A zoom is more optically complex, and mechanically so as well, because of the attempt (not always very successful, back in the day) to maintain focus as you change magnification. If you defocus and refocus after zooming, even the early ones aren't terrible, but they'll never compete with a prime -- and a vari-focus can do about as well as a prime with a factory-matched tele-converter at any given setting (which will blow those early zooms out of the water).
