It's Steam-only, based on a contract between Bethesda and Steam.

But why would Bethesda limit themselves like this?
I can't imagine that Steam paid such a high amount to warrant the loss of revenue of being able to sell Skyrim universally?

I will never buy a game from Steam or buy a game that is dependent on their platform to operate.
Steam sells any crap on the market. Gog is curated and therefore has integrity, providing value to customers.

It's your guess. Money perhaps? :-)
Looks like Bethesda and Steam have done some calculations, estimations and, of course, negotiations and haggling, too, and then signed a contract.
If I were to guess: Steam wanted the monopoly for Bethesda products (we're not talking about Skyrim only, but, for example, about TES Online, too, and Fallout, ...), and Bethesda estimated the number of people who would pay for Bethesda products elsewhere except on Steam to be low - lower than the benefits they expected from selling, supporting and maintaining their products via Steam.

Well, considering the decreasing qualitiy of their new games, I can live with that.

I'm blocked by a GOG forum bug from posting. I can only reply.

I have the skyrim - realtek - 2.1 speakers - 1% volume bug. Need help,,.