Because in that range, sin(x) < cos(x).

Consider a right triangle with acute angles x and y, and with the sides that are opposite them having length X and Y respectively.

As long as x < y, then X < Y.  Call the length of the hypoteneuse Z.  Y is the side that is adjacent to x, so sin(x) = X/Z and cos(x) = Y/Z.  Therefore, if x is between 0 and 45 degrees, sin(x) < cos(x).

q.e.d.