Because Jedi must be celibate. Of course that leaves it open that she's a child of force users who weren't Jedi, but like I said, that's bending the lore around the story again. Are they just going to mention in passing the massive upheaval of lore that would be a Jedi having a child? "Oh by the way, I fathered a child by someone you've never seen nor heard of until this very moment, but this movie isn't long enough to go into detail so let's just forget about it."
IMO, anything coming from Obi-Wan is a massive stretch, though technically possible. They would have to explain how he left the Jedi Order and married someone and had a kid who then had a kid yadda yadda yadda. It would be too cumbersome an explanation.
Don't confuse bending the lore with breaking the lore. A haphazard explanation would ruin this mystery, however possible it may be.