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Some of this depends on the molded part: when injection molding, if at all possible you want to keep the thickness of all the different features of the part ("wall thickness") consistent everywhere on the part. If one feature (e.g. the side of the brick) is 5mm thick and another feature (e.g. the stud) is 10mm thick, it can be difficult to fill the molding cavity and cool the plastic evenly during molding and you might end up with warping, "sinks" (depressions in the plastic surface), and other physical irregularities. I've noticed that some solid-stud parts actually are hollow on the underside to try and achieve this.
Another good reason for hollow studs is the ability to insert the end of a bar into the stud to achieve a certain look during a build...
Of course, that's just one consideration—many other factors are applied in that decision-making process as well, like a part's similarities or differences with existing part families.
I would think that having all bricks with hollow studs gave much more flexibility than solid ones.
Also, as mentioned above branding could be another major factor in favouring the solid studs with the Lego word prominently exposed in each and every stud.