The developers have a page where they explained for users why there is no KyLook for Thunderbird. The main reason is that no one was ever interested to have KyLook working with that application. The second one refers to Mozilla ceasing development to Thunderbird. Now, it doesn't make sense to start or keep developing a product for something that is discontinued. However, there is a way to have Thunderbird working.
Access the page to read about it: http://www.kylook.com/en/faq/thunderbird/