Aside from the standard, sadly misinformed creationist talking points, there is a vast quantity of evidence supporting evolution. Every angle that it's been approached has supported and strengthened it. It is one of the most successful theories in scientific history. But, to the point of why Americans are slow to accept its reality is, in my opinion, due to the fact that we are a very religious country, and evolution specifically applies to our origins. It took the church a long time, and many smart, well-intentioned people suffered, before they accepted heliocentrism. Same with the abandonment of the silly dogma of the perfection and unmoving nature of the heavens. They could eventually accept that and still believe the myth of our special creation and attention from a doting deity. Evolution strikes a near fatal blow to that delusion, and the religious mind lacks the flexibility to adjust quickly and maturely. Also, our science and math education is poor. I would also add that the size of our country contributes to the slow flow of information to isolated, ignorant demographics.