Well the top one is just a crude left-to-right gradient overlay with no regard for the shape whatsoever. You could slap that shit on any old shape and it would basically be the same, red on the left and violet on the right and that’s it.

The bottom one is a nice smooth transition that actually follows the shape. That’s why it’s so much more pleasant. The hue change occurs continuously as your eye goes along the path.

Oh and the bottom one is a more balanced shape overall. The top one feels too tall. Maybe it’s a golden ratio thing, I don’t know.