I think it's because you can accelerate a solid lifter faster without..,

worrying about the problem of compressing the plunger in a hydraulic lifter. It's just metal on metal in a solid lifter. The steepness of the cam lobe is limited by the properties of the metals and the oil.

With a hydraulic lifter, acclerating the lifter too fast will collapse the lifter plunger (I believe). This affects hydraulic rollers, too. They cannot accelerate the lifter and valve as fast as a solid roller and sometimes not as fast as a solid flat tappet.

Solids rule.

Paul