From their blog on why Minecraft was sold:
As you might already know, Notch is the creator of Minecraft and the majority shareholder at Mojang. He’s decided that he doesn’t want the responsibility of owning a company of such global significance. Over the past few years he’s made attempts to work on smaller projects, but the pressure of owning Minecraft became too much for him to handle. The only option was to sell Mojang. He’ll continue to do cool stuff though. Don’t worry about that.
Which I think is perfectly reasonable. He did a thing. It made it big. It stopped him from doing other things. He'll still be the person who created it and brought joy to a lot of people. It is -still- an example of how an originally Indie game can make it big. But no one can expect or demand of him to spend the rest of his life doing one thing if it has stopped making -him- happy.
As for them selling it to Microsoft, Mojang clearly thought that of all the potential buyers, they were going to be what was best for the continued development of the game. If we made Minecraft, we might do things differently, but we don't. All we can really do is trust that Notch didn't make the decision offhandedly and put thought into it.
Here is the blog post, for further reading.
