Why do NOS Tubes sound better than new tubes?

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.

I have heard the same thing about the metals used and that they are not available anymore.

That’s funny, I asked this same exact question to a local high end audio shop nearby. "Deets Sound Room" in Carmichael, CA if you know the shop. I was giving pretty much the same answer as everybody here. But I bet if you ask that same question today to tube makers, will they give you the same answer or will they give you some marketing spill??
I was personally thinking myself tubes made today would be better than the NOS due to manufacture process today could produce a better quality tube. You would think its being built better today, but I guess that’s not what I'm seeing to all the answers. No pride in tube making today I guess.

Don't write EVERYONE that manufactures tubes off, just yet! These guys are making some VERY nice tubes currently: (http://www.euroaudioteam.com/en/products-000002.html) Much can be done with excellence(even these days), IF you take pride in your craftsmanship. YES- It costs!

As far as the sound is concerned, I'll attribute the decline to significantly decrease competition and demand (like many of you have already indicated).

For the build quality, durability, and lifespan, I think it's the shift to a "disposable" society. Cars, lower-end receivers, washers and dryers, telephones, televisions, mass-produced furniture, and so much more just don't last as long as they used to. That's not to say that they're not functionally better in many ways; they just don't last as long. I've had a cheap gas stove from the 50's, and a cheap gas stove from the mid 90's, and while the newer stove is safer (avoiding pilot light issues), the older stove might still be functional in another 60 years while the new one might crap out tomorrow. It should be no surprise that vacuum tubes (no longer built in the UK, Germany, or North America) also have shorter lifespans.