The plain fact is that tape deteriorates! Thus the master does NOT have the sound it had 50 yrs ago. Then there is the lost ones. Plus it wasn't "VINTAGE" then so the records only sold for $2.99 - $4.99. Lots of things happen in 50 yrs. More people went broke in the R&R business than ever made it big, IE Woodstock Promoters.
Why not copy the greats- vinyl LP question
When LP's are reissued, why are some of the great interpretations of classic music not just copied? For example Led Zeppelin II- I would love the RL-"hot" mix but cant swing $500+ for a less than optimal copy. Why is there not someone looking into recreating products like these?