Of course Java project are not immune to failure, that is not what "low risk" means! In this context it means the risk is lower for certain specific problems - e.g. unable to hire developers, show-stopping bugs in the platform which never get fixes, lack of documentation, abandonware.

Choosing a less popular platform (like say Clojure or CoffeeScript or whatever) might very well increase productivity compared to the conservative choice. But slightly increased productivity will not save a project, while the additional risks involved could very well sink the whole project. Which is why most businesses seek to reduce risk rather than maximize productivity when it comes to development platforms.

Imagine adding to the list of risks for a new project the term "low talent pool" and "head engineer leaving".

Many languages haven't been popular long enough to have a nice pool of experienced engineers to pick from. Java has that advantage, which not many languages have.