It's either the field dimensions, practice availability for the other team, or ad signage space. From what I recall reading the regs, concacaf requires exclusive ad space and doesn't want any other ads to be shown during their broadcast that isn't their own. Making the field available for practice may also be an issue too.
You are forgetting the key words in Sims email .. "pending approval". So yea Sims is saying they'd be available, which after crook's question and answer sounds like it means the Yankees are willing to dewinterize it if approved.
if thats the case then its gonna be rough, both stadiums have big signage all over the stadium and they probably dont want more usage of the field for a practice.