The article linked in the OP doesn't explain why the stadium no longer has a sponsored name.
It was most recently named "New Era Field", sponsored by New Era Cap company. That company makes a substantial % of sports caps sold for professional sports teams, and used to have a production facility in Derby NY, which is about 20 miles south of Buffalo.
Since obtaining the naming rights, New Era has closed the Derby plant and outsourced those jobs to other companies. More recently, they took stimulus funding, and then issued over 100 layoffs to Buffalo employees as soon as the funding rules allowed them to do so.
Not surprisingly, local/regional politicians are rather pissed off at this turn of events, and decided to yank the naming rights for the stadium before even starting to look for a replacement sponsor.