Well, it ain't because he played better than Dalton last week - he didn't. Dalton wasn't a problem last week.
Fields definitely has a higher ceiling though.
Dalton was “good,” and the offense was still impotent. There’s no upside to Dalton. This offense isn’t going to go on 3 13 play touchdown drives a game. It’s just not.
Nagy could have helped Fields by calling plays that suit what he already does well. 4 years in, and Nagy still calls plays for the offense he wishes he had, instead of the one he does.