probably because the musicians control the sound equipment, and they have severe hearing loss due to listening to so much loud music over the years.
You need to speak to the bandleader or the sound man. It is possible to have a good sound level. We had a nice four-piece klezmer band at our wedding, and they played unamplified — loud enough for the dancing, quiet enough so you could hear at the tables.
Sometimes they don’t know until you tell them.