I know this is more of an opinion, but I really have no idea!

She really wanted to see and make sure that Orlando is indeed in love with her. Thus she remains disguised as Ganymede in order for Orlando to share with “him” his true feelings about her, after of course she has seen many love verses written on trees by him. Also, she wants to be certain of his real love for her in order to feel comfortable with a man(Ganymede) so she is not reveal in case he may feel awkward and uncomfortable at the very start when she sees her. When she eventually understands that she will appear to him as herself