that the one way of preventing this from happening again was to have a clear, stable succession that nobody could question. A girl just didn't cut it for that.
Well, as I said Henry VII didn't actually say only a son worked. It was the first choice though.
There's another factor to consider: Mary is unlikely to conceive even if married earlier (probably fibroids).
Some women just lose a lot of fertility after the mid 30s rather tan a slow drop. So this might be a case that Mary got the fibroids later in life. Or maybe she just had a shorter window for her womb that most other women.