But to the original question, this is almost certainly because when the original patterns for guitar and bass cabinets were being made, the only speaker choices were off the shelf Jensens, and Electro Voices, and Eminences, etc. They weren’t getting custom drivers and didn’t really understand the physics involved to contract them in the first place. 15” speakers seemed to work better with bass, so keep sticking them in boxes of various different dimensions and see what works.
Ampeg eventually shoved a bunch of 10” drivers in a box and we had the second bass design pattern. Eventually the physics were understood, but patterns and supply chains were already established by that point.