You could actually look to it as a sequential(well, not really but that's how it turned out) progression.
In earlier editions, capabilities were unique to the classes. Martial stuff were practically exclusive to the Fighter, Skill stuff exclusive to Thieves, etc. While maybe not strong, they certainly weren't weak because they could do things nobody could, and much of the best gear were class-exclusive. Their role was protected, but everyone was sort of playing a different game really.
In 3E, martial and skill stuff were absorbed into the core system, but magic was not. This undermines the value of being a martial character, as any character, including skilled and non-martial characters, could be a martial character so long as you could get your numbers big enough. Pretty much everyone could get those. This meant there wasn't really any particular point to adding a martial character, other than making the buff/BFC mages more efficient with their spells.
In 4E, everything was absorbed and homogenized into the core system. Everyone, mage, skilled, martial, was a martial adept as far as combat was concerned. Now there was no particular reason to have a FIGHTER specifically, but there was also no reason NOT to have a fighter.