Why do older pop songs seem to have quieter vocals than modern music?

Answer 1

The short answer is: Compression.

Modern music is more compressed for a variety of reasons, but the common one seems to be because it wants to be louder to sound at the same volume and in-line with other songs. Compression can eliminate a lot of the dynamics of a track, so rather than sounding quieter at parts and louder it at others, it has one general singular volume it revolves around.

Since older production didn't seem to follow this principle, it was less compressed and therefore kept more of the dynamics of the vocals. Meaning the vocals would range from softer to louder throughout the song.

This is just a simplified version of a much greater discussion. I would do a search on: "The Loudness War"