Because: the distance traveled by the wheel = the circumference of the wheel × the number of revolutions.

So: the distance/revolution of the wheel = the circumference of the wheel (certain) (note: if the diameter of the wheel is certain, the circumference of the wheel is certain, that is, the ratio is certain. )

When the ratio of the distance traveled by the wheel to the number of revolutions is constant, the distance traveled by the wheel is directly proportional to the number of revolutions.