I don't know but I can speculate a couple reasons why they might:
-They might not want people who aren't officially affiliated to get "contributor" badges on github, so people don't get confused thinking they speak for the project when they participate in discussions
-github may not be their primary way of managing their source, so pull requests have to be transferred to their internal system first before being released on github
-better control over what gets into what version without having to micromanage when to accept pull requests
Also helps with ownership. They have to maintain the thing, and it's easier to do when the person who committed it is a co-worker you can get a hold of.
-They might not want people who aren't officially affiliated to get "contributor" badges on github, so people don't get confused thinking they speak for the project when they participate in discussions
-github may not be their primary way of managing their source, so pull requests have to be transferred to their internal system first before being released on github
-better control over what gets into what version without having to micromanage when to accept pull requests