IMO it's because when you look at the scope of human history paleo/primal wins out over vegan every time. In traditional cultures you cannot find a single one that is vegan, and you can find plenty that are paleo/primal.
I'm not here to knock veganism if someone feels it is working well for them
. But I don't see any compelling historical evidence that it is the most ideal diet for the majority of humanity, and I see plenty of compelling historical evidence that paleo/primal eating (which is eating clean meats raised on the food they were intended to eat, animal fats, plants, vegetables, nuts, seeds, eggs, etc... (can include dairy, fruits, plant-based starches, etc...) - whole foods with lots of nutrients, protein, fats, vitamins, etc... is a very healthy way to eat.
I don't consider it a fad diet b/c it's not about dieting (in terms of temporary eating habits you do just for weight loss), instead it's about looking at food in a different way and changing habits long-term.