From principles of molding, you want a uniform part thickness throughout if possible. This facilitates plastic flow as well as dimensional stability (you want uniform shrinkage on cooling.) Removing a divot from the underside of a stud serves this purpose in a regular brick. The Technic brick with through holes would have had a large amount of solid plastic under the stud compared to regular bricks, which could create undesirable shrinkage and warping from stress in cooling. So removing the top divot helps there.
However, if you examine a 1x16 beam from Expert Builder, you can observe a plastic channel running along the underside between studs 4 to 13 (which are injection gates; also stud 8, or 9.) The 1x8 doesn't have this. This channel would let the plastic flow away from the injection gate without turning too many corners. It's also possible LEGO used a reciprocal molding action where plastic is pressed in one gate, then another, in a rocking action, to facilitate welding of the multiple injection streams. Solid studs might lessen the effectiveness of this process, but the channel helps it. If the channel was necessary to produce a 1x16 beam back in the day, and the channel forced hollow studs, then the hollow stud style was the only way to make a 1x16 beam. The technical difficulty of making a new, longer brick (beam) may have influenced the look of the whole Expert Builder system.
That's only my theory, but you can still see the channel for yourself.
My tests also indicate the hollow stud separates from another beam more easily than a common solid stud would.
It remains possible though that they were just maximizing the usefulness of the part by making the stud hollow. Though the fun part of being an engineer is maximizing more than one property at once.
So my thought is the hollow stud came from various molding principles. Cost of plastic comes after that (and yes, if you can save a bit, you do.)
Another interesting trivia: the Technic studs were each freehand drafted in the early Expert Builder instruction booklets. I joked about reproducing this while showing some Lego employees (in 2001) what could be done with non-representational rendering, for instance a pen-and-ink renderer with random jitter.