Something else to note is that Delta's B752s are powered by PW2037s which are slightly more fuel efficient but a lot less powerful than the RR engines in AA and UA B752s. According to the detailed performance figures available on the Boeing website, the generic PW2037 powered B752 can be slightly payload restricted opearting out of EDI due to runway length depending upon prevailing weather, runway condition and runway in use. Runway 06 on a warm, wet day is the more restrictive one because of the proximity of the railway embankment and thus the much shorter clearway available to reach a safe height.
https://www.boeing.com/commercial/ai...n_manuals.page
Without access to the exact weights for Delta's aircraft, I can't say whether this restriction applies to them but it's not inconceivable that a more premium heavy version could make sense for Delta if they can fill the premium seats. I expect they will already know the answer to that question.