Many children learn to say "no" before they learn to say "yes".

Because that's what they hear more often?

I think it has to do with the complexity of the word. (Linguistics specialists - HELP!) While NO isn't quite a simple plosive, it's far less complex than the glide/mouth shape change/fricative needed for YES. So, both neural and anatomical development must be a factor in what sounds an infant first makes. (You physicians, please hop in here!)