Search the archives; much discussion. Basic answer is cost of R&D and tool-up not recoverable with low volume market at present.
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What Swampwalker sez, and not all NOS tubes sound better than current production. There is a lot of crap out there disguised as NOS (including a lot of used pulls) and there are some excellent new production tubes. And there is a lot of NOS hype created by those who sell them. You just have to get to know what you need and what is available before you wade in with your checkbook.