There are alot of people with computers on this planet. It's a testament to not only being a great game/experience out of the box, but also the constant updates and the lack of costly add-on content plaguing other games.
I think an important point is also that Minecraft can run on very low spec hardware, no need for fancy GPUs.
Quite a big update with a lot of changes was released yesterday so that probably bumped the sales a bit.