Because he was describing his dream.
Why say something about her when there is nothing in the gospels about her like this?
I don't know what you mean. The Gospels are precisely about this. The mother of a King is a Queen. Jesus is the King of Heaven, is He not?
For example, where does it say that she was in the wilderness for 1260 days?
That is how long she remained in the desert with St. Joseph and her Child.
How would you respond to the pain in childbirth if Mary was supposedly sinless?
The pain of childbirth described in Rev 12 is symbolic to the emotional pain which St. Mary endured in serving her vocation as the Mother of God.
Sincerely,
De Maria