"The golden age" of coloring books is believed to be 1960-s, because almost all characters were depicted in them, as well as some products (like Planter Peanuts or Campbell Soup)
Coloring also takes us back to a time when everything was easier. You agree that such activity would seem more suitable for kids than for adults. In fact, painting is associated by most people with the period when they did not have much to do, and carelessness was more than enough. In order to take a little dip into childhood and clear the emotional background, coloring is an ideal option. It seems to take us out of the current stress state and allows forgetting about everything for a few hours.