Why can’t she just have innate Force powers? Her usage of the Force never felt like a mystery to be solved to me. It’s just protagonist business.
Something something Mary Sue.
To be fair, she picked it up much more quickly than anyone else in the galaxy, including the literal Chosen One.
There is actually an explanation in TLJ for this, the whole ‘darkness rises and light to meet it’. It is shown how Rey and Ben are as evenly matched as can be possible. While this might not satisfy everyone, to expect a different explanation in the final installment is a bit silly. The only thing worse than that would be to actually provide such an explanation, which would alienate those who have accepted the prior explanation as well as coming so late in the story that everyone else has already made up their minds.
In short, pulling that thread in the eleventh hour will just make a mess.