Good question!
It used to be that the mirror makers used speculum metal to make mirrors with. The problem with that metal is that it tarnishes almost as fast as silver does and thus needed to be refinished about every 6 months or sooner. This is a real problem as the surface really needs to be within a 1/4 wave to be decent for optics use. Eventually mirror makers went over to glass with a silver (later when evaporation coatings were developed, aluminum) coating which could be chemically removed without disturbing the glass surface (no more having to generate and inspect the figure of the substrate!).
Mirrors today are mostly glass although the space industry is getting composite (epoxy graphite type stuff) substrates with plastic coatings carrying aluminum or silver or even gold coatings to lighten the load of the rocket carrying the scope up.
Probably the reason that glass is so well loved now is that it's easy to give a high polish to and also easy to figure into a complex surface without any real problems. Metals have been used in the past but they run into the same class of problem as the speculum metal ran into - the polished surface tarnishes and you have to rebuild the shape as a result.

• glass not metal - krispee - September 19, 2001 - 18:36 UTC
  • Glass not metal - bobmay - September 20, 2001 - 18:50 UTC