The ePub rendering code is a mix of Kobo as well as Adobe stuff AFAIK, so it's not as easy as simply relicensing their own code. To release a functioning open-source version they would probably have to cut lots of functionality and expend lots of effort on a release most users wouldn't benefit from.
I'd love it if they did it too, but it's a very big effort, and risk, on their part.
Here's what they do already release under GPL rules:
https://github.com/kobolabs/Kobo-Reader
..and here's a project to build a libre replacement for nickel, the QT app which handles book-reading on the devices. Currently just for the Kobo Touch, but its probably portable.
http://ccoffing.net/tech/proj/ocherbook/