Why do NOS Tubes sound better than new tubes?

Because they were made better.

Before now, I never gave a hoot nor hang as to "why" NOS tubes sounded better. All of the post's have come up with some interesting points. Tpreaves stated that not all NOS tubes sound better. Not all NOS tubes were manufactured for "audiophiles". Many of you act like we're some kind of new breed. Hitler was an audiophile.

Since I couldn't find anything scientific, I came up with theories. The very first "high end" for the public was the "jukeboxes". "Why do you think they sucked in so many nickels dimes and quarters"? It was because they sounded so good. This was due to "high end" parts, including the best sounding tubes. Since there was a large market and demand for "audiophile" tubes, many were manufactured. Those jukeboxes were all over the country in the 40's, 50's, and 60's; and they used tubes, until they went to SS in the 70's and began to sound like crap.

Also Western Electric and RCA in the film industry sought after better sound. Most likely another big profit area.

That would explain why old movie soundtracks sound so much better than any other old audio.

I was told that new tubes aren't allowed to be made with the same rare metals that NOS tubes use. They are heavily regulated and folks high up on the food chain don't want them readily available to anyone who might want to use them for something else.
All that leaves the newer ones with are better engineering, closer tolerances, innovation and cleverness.