It was coached out of him - years of not managing him properly and playing him in too many midfield roles. Unable to put a stamp in a proper midfield role, he wasn't able to develop properly. Davies has been tried in many midfield roles over the years - Attacking Mid, Box-to-box and No. 6 - either by choice or necessity. It would've been better if he sticks to one main role and provides cover for another. It could be also down to him, not being assertive to the coach. Benitez seems to be playing him further up the pitch and we are seeing him making some impact but just flashes, really.
He was never a player with world class talent to begin with but there are less talented players that went on to be really good players. I think if he was coached and managed properly over the past 4 years, we could be seeing a different Tom Davies