hamlet's crazy yo

:Why can't Gertrude see the ghost even though he was her husband?

The ghost was never there. Hamlet made it up as all part of his front. He was pretending the ghost was there to take his mother's mind off of the things he said. The ghost would have more reason to show up in front of Gertrude rather than Hamlet. The ghost in the bedroom displays completely different characteristics than that of the first act. in Act 1 the ghost is eager to talk and stays around to explain himself. But this time the ghost 'shows up' makes an appearance to Hamlet, then leaves.