Why aren't new vacuum tubes as good as old ones?

As Jmcgrogan2 mentioned, when military electronics, including radios, guidance systems and computers, were run on tubes, their reliability and endurance was paramount. Therefore, the tolerances, consistency and reliability of the tubes than ran the equipment were driven to the extreme. Hence, vintage tubes of the 40s, 50s, 60s and even 70s (particularly Eastern Bloc) were of excellent quality that cannot be matched today simply because modern military electronics don't require it, and therefore there's no large scale market to drive tube quality.

I think you guys are under estimating the importance to WE of reliable tubes and the fact that there was competition among manufacturers.

...and the fact that there was competition among manufacturers.
Tbg (System | Reviews | Threads | Answers)

I believe this does without saying. Every tube manufacturer would be competing to produce the best products to win the available military contracts.

I heard that RCA used the same machine to make tubes for 50+ years and every year the fittings got looser and looser so the newer tubes lost a bit of quality each year compared to the original tubes. I forget any more specifics but the general idea makes sense to me (in this instance). This is one part of the equation it seems...

It is my understanding the materials have a lot to do with it, but I believe there is an art to manufacturing tubes and even though the same equipment is used there are different people involved in the manufacturing process.