It wasn't a copy (not really)

That is, it wasn't an exact copy, even though the digital nature of CDs makes this trivial.

"The CD of Slayer’s ‘Reign in Blood’ I bought in the early 90s seems to have way better production value that the copy I bought a couple of years ago"

That's because somewhere in between the first purchase and the second somebody "remastered" the album, i.e., ran it through compressors and limiters and reduced the dynamic range to nothing.