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

Because old ones where made for war...new ones are made for audiophiles...although there are few new tubes that exceeds expectations...

Nobody knows why. If somebody did they would certainly be able to make exact duplicates. 

This thread is very timely.  I own an ARC Ref 150SE amp.  My amp uses 8 KT-150 tubes.  I either read or heard that KT-150 QC has dropped over the years.  I understand that ARC and some of the major tube retailers have a high KT-150 reject rate.

In any case, here's the bottom line.  I recently spoke with the ARC customer service rep.  He advised me that ARC is selling KT-150 tubes for $265 per tube.  ARC charges over $2650 for a complete retube of my amp (2 quads of KT-150 and 4 6H30s).  Yikes!! 

As an aside, several years I bought 2 quads of KT-150s from a well known and respected tube vendor.  The problem is that even though the tubes were matched, the bias settings between the set tube and slave tube were off by a lot.  By contrast, ARC tube match very close.

Any suggestions?