Great question. First of all, coloring is a growing industry with adults. It used to be that only kids loved to color, but now studies have shown that coloring actually reduces stress and is beneficial for adults, too.
From CNN: Just like meditation, coloring also allows us to switch off our brains from other thoughts and focus only on the moment, helping to alleviate free-floating anxiety. It can be particularly effective for people who aren’t comfortable with more creatively expressive forms of art, says Marygrace Berberian, a certified art therapist and the Clinical Assistant Professor and Program Coordinator for the Graduate Art Therapy Program at NYU. “My experience has been that those participants who are more guarded find a lot of tranquility in coloring an image. It feels safer and it creates containment around their process,” she adds. 
From Medical Daily: Art therapy is also helpful among people dealing with a variety of other conditions, such as depression, dementia, anxiety, and PTSD.