Vets recommend Science Diet because it's marketed to them. That's all.
http://www.petmd.com/blogs/fullyvetted/2007/june/how-do-vets-recommend-pet-food-part-1-industry-6142
http://www.petmd.com/blogs/fullyvetted/2007/june/how-do-vets-recommend-pet-food-part-2-education-6141...