Lithium is light. This means that it generally floats to the top of things. This means that it is not a good candidate for asteroid mining, because while it has floated to the surface of the Earth over the course of many many many years, it has not floated to the surface of asteroids.

This is also why things like gold or platinum are good to mine from asteroids, because they have not had the opportunity to sink to the core of those asteroids like they did on Earth. (And getting to the core of an asteroid is a significantly more reasonable proposition than getting to the core of the earth)