Training. Most people who 'can't rack the slide' have simply never learned how to do so. The recoil spring on a 9mm pistol will only be about 16-21 lbs, so you would have to be exceptionally weak (as in, some serious musculoskeletal medical condition, not simply petite) to not be able to learn how.
I have found that some people who are not around guns often have difficulty putting their full strength into manipulating the gun out of a combination of wariness about the gun's dangerous nature and a mild fear of somehow breaking the gun. Treating the gun gingerly will do nothing to make the gun more safe, and I don't think most of us could even physically break a Bryco or Jennings pot metal gun with our bare hands.
Once your wife has the push/pull technique of slide racking down, she should be fine with anything.