It is available on Steam as a DLC package. We've never (so far) had requests for it at our other publishing sites.
I prefer to get it out of Steam. I don't like the idea of log in there every time I want to listen to it. =)
OST's are good advertisement and also nice to include as an extra. Not everyone uses steam, if you are that clueless then look it up. Many people are opposed to monopolies and support you elsewhere, we are on another site after all. You putting it on steam only supports steam. Consider a game that costs money, and an OST that costs money. You can only buy the OST on steam after buying the game. If you bought the game elsewhere and want the OST, you'll have to buy the game again to buy the OST? It's insulting and if any of the alternatives to steam had a spine, they'd tell you as much.
I really wish the DLC was available outside of Steam, as I try to avoid that platform (I buy my games on GOG or Itchi.io mostly) if I can.