Basically, Skyrim Special Edition is the remastered version of the original Skyrim, an open-world action RPG. PC version of the original Skyrim has diverse mods that can affect the game, and some of them are related to making the game’s graphic look better. So, almost all features that are included in the Special Edition were available for PC users. That is why Bethesda has decided to let PC users have free upgrades.
However, the situation is different for consoles. Originally, Skyrim for consoles didn’t offer any chance to use mods, while Special Edition will support PC mods.
So, while PC users does not have any merits to buy Skyrim Special Edition, console users do. That is why Bethesda is not selling it for free on consoles.