There's always that one
There are actually several reasons why stadium food is getting better: 1) stadiums are pushing vendors for higher rents and the only way to get that done, given that they are already serving at maximum capacity, is to sell higher priced, higher profit food, which means, better food. 2) Visitors have come to expect better food and fear novel foods less (could you imagine a char shiu burger in the 1960s?) so vendors are compelled to improve their offerings to satisfy fans and keep their contracts. 3) Franchising is a QSR standard so the the big players (e.g. Aramark) are able to get franchises of already perfected fast food options, which makes it much easier for them to offer quality efficiently. 4) New stadiums are built with food service in mind so they can have the room and equipment to produce, store and serve food. That is particularly true because they need better quality for the corporate boxes, which have now become a major part of stadium revenue.