The main reason is light quality, not just portability. A mirror reflects light specularly, so it mostly preserves the original source’s hardness and direction. If you bounce direct sun or an undiffused flash off a mirror, you still get a hard, focused light with crisp shadows and bright hot spots.

Foil/fabric reflectors usually reflect more diffusely, so they spread the light over a larger area and effectively become a broader secondary light source. That gives softer fill, smoother transitions, and helps avoid obvious double-lighting or harsh shadow edges in portraits.

The efficiency difference usually isn’t huge in practice—roughly up to about a stop depending on the surface and how much of the scattered light is useful. So photographers often accept a little less efficiency in exchange for softer, more flattering light.

If you do want a hard, mirror-like reflected beam, you can use a more specular reflector; mirrors aren’t unusable, just less common for fill because they don’t soften the light.