I have no idea WHY
Some kids just don't like to draw and find coloring boring. It is a skill that helps them learn to hold a pencil and write with more control but you can work on those muscles with clay and scissors/cutting.
How long do you expect him to sit still and color/draw? I saw the drawings most of the preschoolers did in Lindsay's class (4 yo) last year and they were not masterpieces and they were often colored outside of the lines more than just a little bit. They don't have the control to be completely neat about it. Heck a number of 3rd graders in my son's class still were not neat in either drawing or coloring. The other thing I wanted to mention is that with my son, a lot of the time what I considered scribbling in his drawings were actually action moves, such as huge circles showing him and his friend on a roller coaster or a bunch of people driving cars or airplanes down a city street. I started asking him to tell me a story about his pictures and often after he did, I could see what he had been trying to draw. It still wasn't great but it was more understandable and I was able to see that he wasn't just scribbling and wasting paper.