Why are older tubes so much sought after

Trelja- I gave my son my ST70 some years back for his birthday. He ended up with RCAs as Sylvanias weren't available when the ones in it bit the dust. Everything else in the system was different, but- They sounded like an excellent tube(especially the imaging and soundstage). My Cary monoblocks take 6550/KT88 output tubes(matched quads necessary), so- nothing NOS is actually an option regarding price. I'd have to sell a couple ounces of gold to purchase the EAT KT88 Diamonds right now, so- I'm using Svetlana 6550's(latest iteration, ST Petersbug plant) from Upscale Audio. I was really pleased(shocked/surprised) that they actually peformed as good as the NOS Phillips ECGs I had in the Carys. I tried some other current production tubes in them first, and couldn't get them out of the amps fast enough. Off the subject: If you want a quantum leap in the performance of the ST70- Upgrade the coupling caps to film/foil, and install a better power cord. I used a Zu Birth without having to modify the chassis in any way. Those were just two of the basic things(among a host of others) I did to mine that REALLY yielded major results as far as accuracy.

We seek out old tubes because the Gilbert Erector Sets, Tinkertoys, Lionel Trains, and GI Joes so popular in our youth are more expensive and difficult to find in mint condition.

And they're cheaper to ship.

if you speak to many manufacturers of tube amps, tube cd players and tube preamps, in production today, you will be told that replacing the tube with nos varieties will have a mimimal sonic effect. bat and mcintosh are two examples of this.

i have found that many current poroduction tube components are relatively insensitive to changes in tubes.

Mrtennis, all I can say in response to your last post is, bullshit!

Every single tube component from my stable as well as those that I owned in the past benefited from tube rolling. Selected NOS tubes were always my preferences. There ar some nice new stock tubes like JJ or GE 211 among few others but in general, NOS rules. Only my opinion.

P.S
I spoke to more then few designers and in all cases the NOS were recommended and prefered over the new stock. Lowering the production costs and difficulties in finding the right source are just some of the reasons to choose new stock over NOS.