This is mostly an ethical consideration, as it's cheap and effective to open source a badly maintained and tested but functional codebase. The fundamental idea behind the code is very simple anyway - just replace some bytes here and there in CAN messages, as long as you know which bytes and which messages, you can just... do it.
However, there will be some expectation of safety and reliability coming from a company converting hundreds of cars on this codebase. It is quite rare for a fork or derivative project to actually change that much or to actively retrofit fixes in the code down the line. Anything I publish now or in the near future that ends up having an obscure failure mode will likely live on in the field and cause some issues down the line. For my own company, that risk boils down to at most the number of cars modified up to that point. For an open source project, that may suddenly become hundreds or thousands of projects spanning many countries, languages and degrees of calamity.
I'm essentially multiplying my support workload (if I want to be able to sleep at night) without much in the way of compensation for me. I'm very aware of the fact that this project I've started has a long tail. I don't want to be tinkering with 2011 Leafs in 2030 anymore, so I need to think about my exit strategy and any decisions that is contingent on right now. So I'm not publishing any leaf-specific code.
I'm implicitly endorsing... related activities though. As of now, Dala is more or less the only other person in the world intimately familiar with my code and thus able to meaningfully continue where I stop. He's been publishing some reverse engineering and useful data that you can really only know with these kinds of tools and code at your disposal, which I will never try to stop. Likewise, I will be publishing 'dangerous' functionality open source, such as modifications to throttle curves, possible future comma.ai or related library support, etc. etc. Things I can't reasonably upgrade on the Leaf myself but that some people might be interested in trying out on their car for funsies on a closed track. The boring 'this just has to work'-code will remain under wraps though.