I think there are two very basic reasons why DoF scales are no longer put on lenses: zoom and autofocus.
Zoom lenses would have to have dynamic DoF scales that would change for whatever focal length is set on the lens. While maybe this could be done with, say, eink or lcd displays, it's something that's never really been put on lenses before.
But the more problematic issue is what autofocus in lenses has done to the "throw" of the manual focus ring. Old manual focus lenses have huge throws. It's why video shooters prize them. You can scale focus one of these lenses very accurately. Now go grab your favorite autofocusing prime, and test how far around the manual focus ring goes.
If your lens is focus-by-wire, consider there may be no accuracy at all, as the ring is no longer mechanically coupled to the focusing mechanism. And while maybe these rings could be geared to give a longer throw, overall, just looking at the focus scales on most modern lenses should demonstrate pretty clearly that a) there's no room for a DoF scale, and b) even if there were, you couldn't actually set the lens's focus with any accuracy. The difference of a millimeter or two on the ring could lead to a focusing difference of multiple feet. The accuracy simply can't be guaranteed.
Look at a modern lens that does have a the DoF scale on it (EF 35mm f/2L IS USM) if you don't believe me:
You really think you could accurately focus scale this lens to 8m? Or be able to accurately set the hyperfocal point for f/8 or f/16?
And, of course, this leaves out the whole issue of what acceptable sharpness is or the ever-changing circle of confusion brought about by ever changing pixel pitches and climbing pixel counts. What was a given as the hyperfocal point of a lens in film days no longer holds true for digital for many folks.
It is more likely that camera makers would rather implement setting the lens to hyperfocal a different way. OTOH, it's not like Canon's A-DEP mode ever actually worked.