just another fad.
wayyy back when most pet food was still made mainly from beef and horse meat, and wheat as a source of grain, lamb and rice diets came about for dogs with food allergies, kind of like fish & potato or duck & potato diets these days.
of course allergy symptoms disappear if you remove the triggers, and the dogs in question did well on the new lamb & rice foods, and the products became very popular.
as with everything that becomes popular, it's carried into the mainstream. people started feeding these foods not as an alternative for allergic dogs, but just because they thought it's overall a better product.
eventually the pet food industry sees a chance to push yet another product on the market that people will be likely to buy, starting with one company and others following suit.
for a normal dog without any allergies or sensitivities, it's best to offer a food that contains several different sources of protein, in order to provide all necessary amino acids for good health.