People with CS degrees can still harbour misconceptions about programming or computers. That doesn't mean there is no way to understand computer science, nor no reason to try.

In fact, in an ideal world, evolution wouldn't be an important thing for most people to understand. It is disproportionately important because it is the point of attack of fundamentalists against scientific modernity, and a naive view of evolution can leave people susceptible to their propaganda, and therefore more likely to politically oppose science.

"can not understand" is different than harbouring misconceptions. Many doctors harbour misconceptions about various medical issues, medicine is a big field. That doesn't mean they are incapable of understanding the concept once they look into it.

If the standard for accepting a theory is absolute understanding, then the human race won't get very far. I subscribe to various theories about physics (ie gravity). I don't claim to understand them. But I do not dismiss them simply because I lack a complete understanding. I trust those scientists who do.

sago hits the nail on the head to answer the "why". When the People have any sort of influence at all, everything becomes politicized, and those who don't want the system to crumble insist that all voters be educated in an ever-increasing set of things in order to vote sanely about them. To question the need to learn a particular thing that's already been politicized like evolution is to question democracy itself.