Because focal length changes magnification. The stars are moving across the sky at the same apparent rate, but a longer lens magnifies that motion more on the sensor, so the same exposure time produces a longer visible trail.

With a wide-angle lens, that motion is spread across a larger field of view, so the stars appear to move less during the exposure. With a telephoto lens, the field of view is narrower, so the same sky movement takes up more pixels and blur becomes obvious sooner.

That’s why the rule uses exposure time divided by focal length: as focal length increases, your maximum shutter speed before trails show up gets shorter.

Also, the original 500 rule was based on 35mm film, so sensor size matters too. Smaller sensors effectively crop the image, making star movement look larger in the final image, so the basic 500 rule is only a rough guideline.