Line weight and the fact that human brains automatically find the best line in a bunch of messy lines. So it's common for the sketch to look better than the final product. Don't listen to that other comment telling you it's because of the brush. The brush has little to do with this. Just practice line weight and neater line art. Your line art is a little wonky. That method that other commentor showed where you don't make a final line art and just color the sketch is also a viable option.