Source:
PC Gamer
Basically, it boils down to:
- Concerns about a possible Valve monopoly/having too much power over PC gaming.
- Doesn't want them taking 30% of Minecraft's revenue.
- Wanting to stay as independent as possible.
- Thinking about their future strategy in regards to bringing Minecraft to Steam.
Here's Notch's statement in full, so you can interpret it yourself.
All fair points, but I think it's easy for Notch to say these things on top of Minecraft's success. If you're in a position to sell your game without any other DD partners (Minecraft for PC/Mac/Linux can only be bought from Minecraft.net), then why would you sacrifice 30% to a third party? That said, I can understand where they're coming from: Mojang wants to sell their game exclusively on their store. And it's not like they're doing an EA and explicitly snubbing Steam; they're not on any DD stores for PC games.
Honestly, Minecraft has enough momentum for them to never have to sell it on a third party store, so I don't expect that to change. But what I'd be interested to see is how Mojang sells games which aren't Minecraft. Maybe they'll find that they need Steam, Origin, GMG and whatever to get Scrolls or 0x10c to sell.