There is single track on the Foxboro spur except for right near the Foxboro station, so the logistics of multiple trains can get complex, especially if you were also running them from Providence. Not sure the minimum frequency you could get, but that limits the utility of extra trains somewhat.
That said, concert-goers could wait half an hour or more for the next train and still probably beat the traffic out of the stadium. I am guessing the T simply didn't expect this level of demand and it was too late to change plans. It is certainly much more demand than any other concert, which makes sense considering the makeup of the Swift fanbase, but it's not like this has been a major issue before.
The Kraft group doesn't really have incentive to promote train service very much either, given the money they make on parking, and it's not like the shows aren't going to sell out if people have to drive or carpool. They only funded weekday service because the parking is sitting there unused and they want to get people going to Patriot Place etc. It would be nice if they could be cajoled into funding a study or even extra service for weekend events at least.
