It's a fad because you draw your "evolutionary diet" line at the Paleolithic for no reason at all. For 100 million years prior all your ancestors ate a pure vegetarian diet. Why isn't that diet more sensible from an "evolutionary" perspective?
So what? Vegetation diet 100 million years ago I'll guess that obesity is nill comparatively to today.
I'll also guess that there was comparatively virtually no obesity in the Paleolithic period.
Nowadays we live in a period where we're told pasta is healthy, or orange juice is healthy (a lot of brands have more sugar in the juice than coca cola). We all eat lots of bred, refined carbs. Sugar has never been so readily available. Parents buy their children sweets all the time.
And guess what? We're turning into a developed world of obese people and higher levels of heart disease.
So yes, a modern vegetarian diet probably is healthier and safer than sandwiches, crisps and orange juice.
Paleo diets are not fads. Our bodies are tuned to metabolise those sorts of foods. People relentlessly defend carbohydrates when they are the most likely candidate for the epic levels of obesity and other related diseases.
Basically, the response to the change of the environment of a species depends on three factors:
1. Heritability 2. Intensity of the selection 3. Number of generations that selection acts.
What this means is that 10,000 can be more than sufficient to fully evolve the ability to eat and digest dairy and grains.
I strongly suspect that my excellent reaction to low carb diets is because I haven't evolved the ability to consume them as well as other humans. Clues include oral allergy to birch pollen and my 'output' when I eat dairy and grains is less than optimal.
I don't believe low carb works for everyone. It doesn't work for my wife. But it is a useful tool, that I'm very glad I found.