Why do you think shakespeare chose to use apostrophe rather than simply describe a menacing dagger?

By having Macbeth directly address the dagger - - rather than just describing it - - he highlights Macbeth's unsteady mental state.

It is one thing for Shakespeare to describe a dagger; but to have Macbeth speak to it, while recognizing it is not physically there - - that's something else entirely. Thus, the apostrophe best conveys Macbeth's internal battle as doubt, fear, and willpower all exist in him at once.