Well, you never really could in any of the games in the series. You'd just find them in the fields and caves, in the form of obtainable items.
My best guess would be, if you could buy potions AND Ethers, Elixirs, etc, then less people would use Pokemon Centers in-game. And of course, Game Freak wouldn't want that.
Or Game Freak knew that people would just just continue to spam low PP attacks like Fire Blast, if they decided to make Ethers available in Poke Marts.