I have several possible explanations. In no particular order—-
1. Master tapes have deteriorated in the decades since the recordings were made. The records I listen to are mostly from the 1950s through early 1980s so the original tapes are as much as 70 years old!
2. The quality of vinyl used in pressing records may not be the same quality as 50 to 70 years ago.
3. The original recordings were made with all-tube recording gear and pressing lathes. In addition, the tube gear used carbon film resistors and good sounding capacitors, what we would now call “vintage components.”
4. The original recordings and pressings were made before the power lines were polluted with all of the RF hash that we have to live with today. Also, they were made before microwave towers, cell phones, WiFi, etc. Today we live in a soup bowl of RFI that undoubtedly has some effect on all new pressings even when the master tapes were made 70 years ago.
5. Recording engineers in the 1950s through early 1970s may have been more attuned to “natural sound” to coin a phrase than modern engineers who have grown up with multi-miking, 24-track mixers, solid state recording equipment and Yamaha studio monitors instead of Western Electric, Altec and Tannoy monitors.
I am sure there are other factors but those are the ones that pop to mind first.