MIT license is a very simple license that basically has no restrictions which makes it perfect for a decentralized project such as Bitcoin that doesn't want any obstacles in front of its development, improvement and adoption.
This way you can create proprietary software on top of MIT licensed Bitcoin without restrictions but if it were something else like GPL they would have to do everything from scratch.
Here is what Satoshi said:
If the only library is closed source, then there's a project to make an open source one.
If the only library is GPL, then there's a project to make a non-GPL one.
If the best library is MIT, Boost, new-BSD or public domain, then we can stop re-writing it.
I don't question that GPL is a good license for operating systems, especially since non-GPL code is allowed to interface with the OS. For smaller projects, I think the fear of a closed-source takeover is overdone.