There are over 600 colors in this piece. It's well constructed but because there are so many colors, it's not really subject to the techniques of pixel art. I do not think these colors were hand chosen. The limited and carefully selected palette of pixel art is what gives it it's crispiness, that good pixel feel. It's not that you're doing anything wrong, just that the critiques on this forum are about pixel art techniques.
I was going to do a color edit, and add contrast to the faces to try and crisp everything up, but it's impossible with all these colors.