Probably all about thermal expansion, which need to be as low as possible to preserve the shape and thus focal length (etc.) of the mirror.
Compared with Pyrex, aluminium has about seven times higher 'thermal coefficient of expansion' (if I got my calculations wrong, it might be 70 times!), and ordinary glass two to three times. Pyrex also has a low 'thermal resistance' ... presumably this is a good thing.
Speculum metal was used in telescopes until the mid 1800's but tarnished easily:
http://en.wikipedia.org/wiki/Speculum_metal
Also I seem to have heard that mirrors have been created by rotating large pools of mercury which then form the desired paraboloid shape (under hi-tech computer control I suppose).
That's about all I can contribute about metallic mirrors!