Really low rings used to be far more common because many hunters wanted to be able to also use iron sights, because scopes often fogged or were otherwise unreliable. Most rifle stocks were still designed (and some still are) primarily for iron-sight use, so for the best scope alignment, they needed to be mounted low. (This is why also why Weaver rings were designed to be easily detachable and replaceable.
Today most factory hunting rifles don't even come with iron sights, so tend to have higher-combed stocks, and aftermarket stocks tend to have even higher combs. Many stocks also have adjustable-height combs, so the shooter can tweak it, rather than trying to match scope height to comb height. Plus, many of today's scopes won't even work in really low rings, because their objective and eyepiece bells are so large.
As a result, there's not as much demand for really low rings. In fact one of my gunsmith friends, Charlie Sisk, has found that most of his customers are better off with medium-height rings.
