https://technology.mlblogs.com/an-introduction-to-expected-weighted-on-base-average-xwoba-29d6070ba52b
A really good read and helps explain. In short though, ground balls are much easier to predict what will happen based on exit velocity and angle because there's fewer Xfactors in play, fly balls have higher error margins, due to wind, sea level changes, long or short porches etc.