I trained in Tang Soo Do Moo Duk Kwan from 1979 to 1982. The intervals between belt promotions were 3 months, starting with white belt. I do remember spending at least a month working on stances and basic blocks before throwing my first punch, then learning punches and my first form in the second month. Probably didn't throw a kick until my third month.
Never the less, I still tested for green belt at 6 months, and red belt at 15 months. I was a red belt for another year, and made Cho Dan Bo, at somewhere between 24 and 26 months.
So even back in the day, from a traditional Korean instructor as part of the same organization you were part of, people weren't kept at low belts for years. I suspect the reason is, you train for years for one or two belt promotions, a lot of people would get bored and quit. You have to get into the curriculum at some point, hopefully sometime within the first year of training.