Why are multiple marriages becoming so rare?

It seems to be getting less and less common these days. The days of marrying a young girl as a second wife are over..It is unfortunate. What changed? Why is it not accepted anymore? My second wife is 17 and I'm 52 . I have a friend who is 49 and he married a 16 yr old girl as a second wife. Other than that, I don't know many men with second wives.