In part because the events seen in Black Summer take place many years before what happens in Z Nation.
It’s likely there are many viewers that hoped to find some connective tissue between Z Nation and Black Summer beyond just the central zombie outbreak that the characters are all facing.
So while it’s possible that Season 2 could somehow feature Harold Perrineau’s Mark Hammond in a sequence, it’s not set in stone.
However, it was confirmed ahead of the release that no such character-based crossover plans were in the works.