Silvering is a nice reflector but it has some severe problems. When exposed to the air, silver tarnishes to silver oxides and sulfates. None of the oxides or other results of chemical action protect the silver from corrosion.
Thus the average life of a silver surface is measured in months in a decent non-smoggy area.
On the other hand, aluminum can be coated with aluminum oxide which is transparent and provides an inpenetratable layer over the aluminum. This protects the aluminum from any corrosion and note that if the surface does get scratched to the aluminum layer, the aluminum will self-heal that scratch to a fair degree.
In addition, after the aluminum is put down on the glass, all kinds of other overcoatings can easily be done over the aluminum to protect it and enhance the reflection of the coating while the mirror is still in the vacuum chamber.
Silver, being a chemical deposition process, needs to then be put into the vacuum chamber to obtain any further protection.
Even in a smoggy area, the life of a vacuum coated aluminum surface can be measured in years or decades.
Yep, you can do silver and get about 4% more reflectivity but that life is so short in an atmosphere is going to mean that you're going to be constantly replating the surface.