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

"new vacuum tubes are not as good as old ones "
That used to be my line of thinking. NOS tubes were made better, better choice of materials, more consistent, etc.

After trying the latest PSVANE Treasure series, not sure I would agree with that original line of thinking anymore.

The sonic qualities on the T-II PSVANEs are so good, I actually prefer them to many NOS types.

this would include input tubes as well as rectifiers. The VANE 5U4G, is an excellent sounding tube,....

Audio components that measure the best do not necessarily sound the best. There are many aspects of music reproduction that we haven’t figured out how to measure yet. Tests that objectively prove what we should like just don’t work in audio, it’s a very subjective pursuit. Can anyone prove that people should prefer Picasso over Van Gogh?

If one prefers current production tubes, that’s what he should buy and enjoy. If others prefer classic tubes that’s what they should buy and enjoy. That way everybody is happy.

So mr_m 
your answer is “no, I have no data or evidence” 

Agree tomcy6. Some people prefer components that distort more (measure poorer). I was defining better as “distort less”. 

My Yamamoto A08s uses 45 tubes.
I have both the original WW2 era NOS set, hand / ear picked and matched by Mr. Shigeki Yamamoto himself, and a set of new production EML mesh 45s.
The EMLs are 3 leagues above the NOS in terms of sound quality in every aspect.
Yes, they do cost > $700 for the pair, but they are worth every penny :-)