How much work could it possibly be to simply deprecate the software, let alone open-source it? Flipping the switch to break every existing device is frankly malicious.
Some discussion I saw elsewhere speculated that it was to avoid liability since they won't update it anymore, although there's no reason they couldn't open source it as an at-your-own-risk way to keep using it (or at least provide some documentation to the people already jailbreaking them).
