It was largely unknown, then became briefly popular among health nuts and hippie-yuppie types, which some businesses took advantage of to overcharge for a super cheap crop. It became a symbol of overpriced healthy foods marketed to suckers.

Pretty much this. Kale is effectively wild cabbage, it's a salty leaf that is common in Europe as garnish or a side dish (usually boiled). It pretty much can grow anywhere it's an extremely cheap crop to produce and it was 100% marketing fad that capitalized on the ignorance of consumers.