Because fans have little trouble getting to the stadium by car. The games sell out. There are doubtless many people who head to the stadium just for the tailgating and don't even have tickets. What "more business" there? Your rail spur would cost money to solve a problem that doesn't exist.
As for making money--the Bills provide Pegula a risk free massive return on his investment and a steady income stream.
As for using the stadium for other events, the owners of the stadium are free to book any event that will go there--exactly as they do now. They are free to fill it with "more business" if they can. The truth is there are few large stadium events to book. No one is going to build "a mall" out there. Malls are terrible investments---dead.