Why do NOS Tubes sound better than new tubes?

They kept the machine for several decades so tube quality at the end of the machines life was relatively poor.

Could explain why there are so many complaints about new records people have to keep returning.

A lot of NOS tubes sound bad. There are good ones and bad ones. I prefer the Winged C 6550 to the GE 6550 for many reasons. First, the WC sound better, less shrill treble, better bass, and more reliable. I do like the Amperex BB 12AX7's. But I hear from friends, they have replaced theirs with the GL 12AX7's and like them as well or better. I think new tubes can be very good and just be open minded before spending 300.00 for a rare NOS that may not be any better.
Tpreaves is asbsolutely right.

Better trained people and better materials back then. As far as one tube sounding better than another, it depends on the application.

They were also allowed to use hazardous materials and work processes that would never be allowed today.

Back then the Military had a great need and relied heavily on ruggedness as well as quality. Can't remember where I read it, but the article stated that they spend much less time creating the vacuum in modern tubes. Back then, an hour on the vacuum pump, now 20 minutes. I never had a nos tube that was dated before 1940's and they were great. If quality was waning then, hard to imagine how good the tubes from the 20's and 30's sounded.