And please, for the love of god, don't do any "only X" diets. Just eat balanced and go outside every now and then, and you'll be okay.
Hot take: If it were that easy, obesity wouldn't exist.
People are different, their metabolism works at different rates, they might have sensitivity to different foods, and not everyone has the chance or resources to solve the likely psychological issues related to the condition (either cause of, or caused by) - which are often the hardest ones.
Diets can provide some direction, and are useful in a number of situations. You can't tell a prediabetic person to "just eat balanced", because that will not work - they need a precise diet to survive and possibly revert the condition.
Guess what; no one can agree on what a balanced diet actually is. It's quite literally one of the least scientific concepts ever invented.
In fact, there's evidence to the contrary that eating a "balanced" diet that combines macronutrients really is not a good thing.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2739696/
https://en.wikipedia.org/wiki/Randle_cycle
The potato diet likely works because fatty acids and glucose aren't competing to such an extent that cells stop accepting more energy, thereby causing the body to have to release more insulin. And because potatoes are very satiating.