I think it's because an awful lot of folks can't sustain a low-carb diet long term. I know some do, but a lot don't. Same with low-fat diets and low-cal diets. I remember back when the "trend" was to go ultra low-fat before low-carb. Before low-fat, I remember all my dieting friends counting calories -- plus you've got the cabbage soup diet and grapefruit diet in there, too. So many folks can't sustain these types of diets for the long haul, the diet runs its course, and the media/book publishers need to move on to something else to capture the audience.
It should be interesting when high-fiber diets get their turn in the spotlight. Hmmm...might be a good time to invest in some Beano shares and begin to stockpile purse-size mini air fresheners
Lysne