That is not a "fact" that doctors and science agrees with. Can you prove that "fact"? Why does diet not work for thousands of other people? Why do some people have horrible diets and no acne?
Genetic differences. No one disputes this - different people have different susceptibilities to developing acne. However, that doesn't mean that these genetic differences are the cause of acne.
What people are saying is that environmental factors can trigger genetic predispositions.
As for diet changes not working for everyone - sometimes damage takes a long time to heal. And there is no one size fits all diet that will simply cure acne. These two factors combined make it very hard to solve the problem. You can think that you're doing everything right because it worked for someone else, when in reality you're doing the opposite of what's right for you.
I think it's also important to note that acne is a multifactorial disease; simply 'improving' one's diet (which can mean anything from veganism to paleo) is often not enough, in part because food sensitivities are many times caused by some unknown underlying factor which can again be very difficult to identify (bacterial dysbiosis, specific food intolerance that results in damage, vitamin deficiency, parasites etc).
Finally, you should understand that people usually only turn to the 'natural' solutions when conventional medicine fails. There is an inherent population bias within this group; these people often have more severe cases of acne that may be caused by underlying conditions that are more difficult to treat. This can make treating acne with lifestyle changes more difficult. Alternatively it can make it more simple, depending on which way the bias swings (this group of people might share a common cause of their acne, which is why it might not have responded as well to conventional treatments). But either way, lifestyle changes often help where conventional treatments have not, and lack of evidence for the success of lifestyle changes in mainstream literature is not evidence against.
(Although the pool of evidence continues to grow)