This is an interesting point, and one that I think speaks to the changing nature of character classes themselves over time.
When classes had less of what we now call "class abilities," I think they worked in a much broader range of applications in terms of what function they were able to serve. While you still have divides between who has more hit points, more combat ability, and the major issue of whether or not you can use magic, there's a lot of space within those distinctions. Your fighter can still function as a scholarly character, a wise political leader, a (mundane) doctor, etc.
If you start adding a lot of special abilities that are related directly to fighting, however, that potential breadth is narrowed. All the more so since there's likely to be multiple other classes (or at least sub-classes) dedicated specifically to each of those other roles.
It's essentially a question of specialization at the mechanical level, I think. To extend that idea, the more mechanical "heft" (for lack of a better term) you give to character classes, the more they tend to become a certain specific thing, and the less they can fulfill other roles.