My wife claims its because silver is more expensive, but I don't believe that. Because it is easy to vacuum coat it and easy to protect it against scratches and chemical problems. The silver used in the past was chemically deposited from solution, and had to be buffed to get a good high reflectance. It also scattered much more. No, that's not it.
Silver is much harder to vacuum deposit.
And it, of course, tarnishes. So an overlayer is needed to protect it. Fluorides such as magnesium or thorium fluoride are usually used. These work well, but are more fragile than the quartz coating used on aluminum. They also are extremely good in the visible, but are extremely expensive.