Because they rework for a reason. Either the champion is unhealthy for the game or the game is unhealthy for the champion. Make Galio Y a new champion and Galio X will almost never be touched again due to a superior design on Y as that one is modernized. Sure X may have a dedicated playerbase, but overall, will remain at the butt of jokes. Only a handful of players will actually play X, and it only serves to hurt the game in reviews of "Why the fuck don't they buff Galio X, no one plays him" when numbers aren't the issue, it's kit design. That's why they rework by the way. They always try numbers first. If numbers can't fix it, a rework is necessary.
Same for oppressive designs. Rengar X was a fucking blast I'm sure. I can't think of anything more fun for a Rengar player than ulting, and one shotting a champion with 0 counterplay at all. I'd love to be that Rengar. Idk if you were around, his ult used to have no indicator. As in, no ! Or screen shading or anything. You'd simply sit there and quite literally out of thin air Knife Cat would materialize already mid air half way to you ready to one shot you. But for anyone else? Terrible. Needed a rework. Hence why Rengar Y (well I wouldn't consider adding the ! To his ult a rework, moreso his assassin update was a rework) is healthier, although still has some issues. They're ironing it out.
Sion is my perfect example of oppressive design actually. Not sure if you played vs old Sion but he had an insane AP play style mid that was sorta niche but holy god was it terrible and had about no counterplay in lane. I won't explain it all, on a phone and there's a lot to how the kit synergized, just look it up on YouTube or in archives, but he essentially could one shot about anyone with a combo of a ranged,two second AP scaling point click stun a MASSIVE AP scaling shield that exploded much like his does now on a low cd. Again, many people would love that. I personally wouldn't mind ruining some mid laners day by doing that and not allowing them to play the game. It sounds very fun. But it was terribly unhealthy. His options were build like a shitty top lane AD off tank or go a cheesy and simply unfair AP mid build. Sion X was shit and was considered garbage tier. Then they made Sion Y who I believe nailed Sion's theme perfectly and saw so much play and still continues to. Sure, Sion Y could have been a stand alone champion with maybe a minor tweak to his shield and scaling health mechanic, but That wouldn't fix anything. That would just give riot a reason not to release a brand, brand new concept of a champion as well as still not address the issue of a broken champion.
However I'd be for allowing the use of old states of champions in custom modes. That would be cool. I'd love to give DFGaren a spin (hahahaha kill me now, I deserve it) or something like AP Sion or maybe old Kassadin or old AP mid/top Soraka a try. But I don't, along with most I'm sure, want to see them in my regular LoL games. Especially ranked.
Basically, TL;DR, you spray perfume on shit, and it's still shit.
