Was the use of silver on 'optical' telescope mirrors more common in the past? Why?
Aluminium coating is a relatively recent process - it became available around the 1920s or 1930s. The Hale telescope arrived just in time to take advantage of this new technology.  (It requires a reasonably good vacuum to work, which probably explains why it took a while to come along.)

Before that - around the mid-1800s - various chemical "silvering" processes, usually involving real silver, were used. These weren't ideal as silver tarnished over time - the silver literally oxidised on exposure to air.  (Aluminium also reacts with air, but forms a thin oxide layer which doesn't harm the image and prevents further oxidation.)  Also some of these silvering processes were either toxic or explosive in application...

Before that - the first telescope mirrors were made of metal that was cast and ground to the required curve then highly polished. The material was "Speculum" a copper-tin alloy. They were poor in quality; far less reflective than later mirrors.  This account gives a nice description of casting mirrors for the Parsonstown telescope using over a ton of molten metal. Two mirrors were necessary for the larger telescopes because the mirror in use had to be taken out and re-polished so often...