Is it something like Big Kale farmers having really good marketing?

Yes. "Superfood" is a marketing term that seems to originate with the banana industry, which ballooned massively in the mid 20th century due to the violent expansion of the United Fruit Company. To sell the glut of bananas that came into the States, they sold it as a "superfood" and a great addition to breakfasts. The term really caught on in the early 2000s/2010s, probably as a response to the development of "wellness" as the new face of diet culture. Unlike diets of the past that were more obviously designed simply to cut calories (like the infamous cabbage soup diet, the Master Cleanse, the grapefruit diet) where the only criteria for "good" or "bad" food is calorie (or macro) count, under wellness culture, certain foods are granted near-magical qualities to heal disease or extend life. This usually follows other trends like anti-oxidants (pomegranates, blueberries), any kind of exotic-seeming traditional food (goji berries, "ancient grains"), and probiotics (yogurt, kimchi).

Kale specifically was brought to the limelight in a marketing push by Oberon Sinclair, founder of the marketing agency My Young Auntie. Some research reveals the bizarre fact that she seems to have done this entirely of her own volition with no support from any kind of kale farmer's organization.