One recurring point on HN in recent times is that there is a major talent shortage. It is very possible that Facebook is not able to find skilled Python developers to work on the SDK.
FriendFeed, acquired by Facebook in 2009, was a Python shop. These guys now maintain their Tornado platform as Facebook (https://github.com/facebook/tornado). Facebook's CTO, Bret Taylor, is a Python guy (https://github.com/finiteloop).
Python is used in house. They have the resources to support Python. They've just made the business decision not to.
Having Python experts on staff is no indication that those people have time to work on additional Python projects. And if there is a real talent shortage, it is quite likely they cannot find anyone else to take on the extra projects.
Ultimately, you are right that it is a business decision. They could hire anyone off the street and train them to be skilled in Python. Though I understand why they might not want to do that.
From everything that I've read and seen first hand in doing interviews, the reason for the major talent shortage is because FB, Goog, etc have first dibs on the best people. What I suspect is more likely is that their general interest primarily lies in languages other than Python.
FriendFeed, acquired by Facebook in 2009, was a Python shop. These guys now maintain their Tornado platform as Facebook (https://github.com/facebook/tornado). Facebook's CTO, Bret Taylor, is a Python guy (https://github.com/finiteloop). Python is used in house. They have the resources to support Python. They've just made the business decision not to.