I've used mirrors for along time and find them really helpful. You can see cars coming from a long way back and see their road positioning as they start passing you so you are forewarned of close passes.
They can be negatively affected by the rain and the dark but then hearing can be affected by speed/wind and traffic noise.
Part of the reason I like them might be due to needing prescription glasses - it's uncomfortable and slow to turn my head far enough to see far behind me through the lenses.
That said, they are a huge pain in the arse. To be useful they have to stick out beyond your handlebars so they get caught going through doors and gates and they adjust themselves when you lean the bike against something. These are not issues with other vehicles. All the mirrors I've tried that sit 'within' the handlebars are useless as my body blocks the view. They also interfere with bar ends. Perhaps they work better with drop bars.
I can see why more people don't use them and they absolutely should not be compulsory. Ultimately they are not necessary for bicycles in the way they are for cars since you can turn to look wherever you want. If they were mandatory people would just stick a tiny unobtrusive one in the middle of their handlebars that would do nothing.
