I'm not sure that "most" is a fair assessment of the number. I've encountered far too many creationists who deny even that.
Even the ones who do accept it have mental blocks against evolution in larger forms, although they have trouble agreeing on *what* amount of evolution is allegedly impossible.
But the point is that a full understanding of the biological sciences requires far more than just an understanding and acceptance of "microevolution" -- evolution of all types bears heavily on almost every aspect of the biological sciences.