In the earlier generations of Pokémon, when the Exp. Share was first introduced, it was used specifically as a held item. Give it to a Pokémon, and they gain battle experience even if they aren't used in the fight.
So why the drastic changes all of a sudden? I'm not complaining, per se—the Exp. Share has benefitted me substantially, and I love the way it functions now—I'm merely curious as to the changes in the game mechanics.
Is there any particular reason as to why the Exp. Share has been altered so much from the early generations to XY and ORAS?