Public money becomes another consideration. Boston, and the citizens of the area (blessedly) have not been in the business of giving charity to sports owners. While the Red Sox can surely afford to finance a XXX,000,000 dollar paradise, would they want to?
Also, usually what drives the need for stadiums (according to Judge Smails) is not seating - it's not season ticket holders, it's not normal gate - it's luxury boxes and club seats. If Fenway is maxed out there, then a new ballpark is at least on their radar.