Why do old tubes sound better than current tubes?

just wondering, is there something in the design, materials, or fabrication that makes old tubes sound better than those being currently produced?

it seems nearly universally held that old tubes are superior sounding to those made today - is there something specific about the old tubes that make then sound better?

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I have a rather LARGE collection of NOS and "pulled" tubes both preamp and power tubes bought most in the 90's. I 100% agree w/ orpheus & Virdian. I originally collected vintage guitar amps from the 60's (mostly Marshalls) before I got into i-Fi again. In my experience ANY new tube does NOT compare w/ vintage tubes,period! Now I'm just talking preamp tubes. I will attest to the fact that when it comes to KT88 and Kt66 tubes some of the new ones beat out my vintage ones. I must stress that I'm comparing them to vintage USED.
When it comes to NOS and even used 6L6's/5881's, El 84's, and EL 34's..... NO COMPARISON! the vintage win! Especially true w/ EL 34 mullards and telefunkens. I would not even "think" to put in ANY of the new variants available when it comes to them. My 02 cents...
Viridian, Opheus10, Tjnif, others...

Regarding some of the more commonly used preamp tubes, do you find that "joe's tube lore" remains a valid tool for winnowing/choosing vintage pulls etc when preparing to do tube rolling?