This is could both be an excellent question or a less interesting one :p

The less interesting one is answered that by Being you are, and thus cannot become. (Just as u/toofpic describes)

The excellent question is why we can't Become when already Being, or why we can't unBecome from Nothing. For that we'd have to read more Hegel to understand the context.

Why is it that when you take away from Nothing, nothing happens? It makes intuitive sense, but why must it be so? What would happen if it weren't so?

Hegel suggests that the qualities of purest Being and purest Nothing are the same, and that's why the Becoming takes them into eachother.

I don't know that that's useful though, someone else care to explain?