Isn't Mojang AB a huge studio with a pretty decent budget, by the time I'm writing this? Why the slow pace? I know he's still indie, by some extent, but still...
Because Notch, a long time ago, decided if he's going to do this, he's doing it his way. Whatever his reasons, I can't really fault him on it. At this point he's successful enough to be begrudged a slow pace for his work. And Mojang is pretty big, but it's still only in the Tens of employees, if I remember correctly, and they do have three other games they're working on at the moment as well as who knows how many side projects and merchandising opportunities... and who knows how many "technically" useless* people like Minecraft_Chick.
Notch does things his own way, and if that means laying out the skeleton of a project by himself before letting people come in, clean up, and fill in the details, then that's how he's going to do it. Honestly, I don't even connect Notch with the rest of Mojang at this point, except as some sort of vague figurehead. Notch is Notch. He makes game, by himself. Mojang is Mojang. They make games as a group.
*I say that with all respect given. She's not going to be making games, or doing much to be making money, but the PR she does tends to be pretty good, and she's apparently a pretty big help with the cons and I have much more respect for her since they started in on that Africa charity thing.