Mainly the pet food companies are like drug reps for the large pharmaceutical companies that go around getting doctors to prescribe their drugs. The reps tout the benefits of their food and the vets get a discount on the food which is sold at a huge markup. Many vets don't get a well rounded education on pet nutrition in school and don't do their own research (which is honestly understandable because veterinary school is a lot of time and work in itself, so who has the time to start doing their own research?). The big reason for this is there aren't many unbias educators to teach nutrition in the schools, so the vets have all the information that the big name pet food companies put out beaten into their heads. If you notice, most nutritional studies are done by big name pet food companies because they have the money and incentive to perform those studies. There aren't really that many independent unbias studies done on pet nutrition.
I do understand why some vets are hesitant to recommend foods outside of the large name pet food industry. It's because they're wanting to recommend foods that have been fully researched and have scientific studies (albeit bias) backing their nutritional benefits. If the vets had more unbias information to go off of I think they would be more likely to recommend better foods. There is just not the amount of independent research on pet food nutrition that there has been on human nutrition.
To be fair, us owners are also shooting in the dark because we're going largely on anecdotal evidence and the small amount of unbias research we can find. We could be wrong on some of our info also, but we need more research to make fully educated decisions about our pets.