Question

why the motion of a wheel of a car on a straight line is rotational and not circular motion

Answer 1

'Circular motion' describes the motion of a point on a circular path.
'Rotational' motion describes the collective motion of a group of points (here a solid).

When a solid rotates (rot. motion) about a fixed axle, then all points have a circular motion, and the words seem to be interchangeable.

But when the axis is not fixed, as in the example of the wheel, is is wiser to use the correct word, depending if you are talking of the solid or of an individual point.