Why do you say this? I have not read that before. Hill had acquitted himself quite well to that point except for not being able to get along with his superiors. He fought hardest in the 7 days and was the hero who saved the day at Antietam. Ewell was a good fighter with a good rep but had been pretty much out of commission since losing his leg at 2 Manasses and was, as such, a bit of an unknown quantity.
Because Ewell was senior to Hill, had recovered from his Second Bull Run wound, and was ready for another command.