They did. In gen 1 it functioned like it did now, but in gen 2-5 it functions where a Pokemon has to hold the exp share item for it to have it gain exp from a battle in a battle it did not participate in. The Exp Share in gen 1 worked like it did now a days, but it was very glitchy so they changed it in gen 2, since a few gen 1 glitches still occur in gen 2 like cloning, exp under flow, etc. so mabye the exp all glitch was also present in the gen 2 betas or something so they just changed it. I dont know why they never changed it back, but once in gen 6 I guess they realized a lot of tiny tiny 4 year olds play their games so they wanted to make it easier.