Its much easier for the general public to eat a varied diet by not restricting themselves to veganism.
A varied diet is often promoted because it gives you the most chance of getting the nutrients your body needs, but it fails abysmally when it comes to excluding things your body doesn't need.
If you also start to consider the environmental impact and animal abuse that comes from 'just eat everything', then it becomes very compelling to put a small amount of effort into reducing meat/dairy consumption and aiming towards a whole food plant based lifestyle. Small steps aren't difficult for most people.
Which nutrients (micro or macro) would you say most people would be missing if switching from a omnivore diet to veganism?
The other thing which I believe (but no idea if scientifically true) is that different people (due to genetics or whatnot) respond to diets a bit differently. A high carb diet for me is a no-no. I get joint pains, tired all the time, break up in acne all over my scalp (and no, I don't eat grains. I eat rice or pasta maybe once a month, most/all is from veggies and fruit). If I go over 100-150g of carbs I have these effects.
My ex-wife had issues with a high protein diet that exarcebated her anxiety and panic attacks.