Emil,
A few reasons, the most important is that sound quality should be better with fixed bias (less complicated).
With fixed bias, you have to reset the bias every once in a while as tubes age and it is reported to sound better. You don't need a large cathode bypass cap (sometimes a crappy electrolytic) in the AC current path (signal path) as used in Auto Bias (cathode bias). Some tubes require auto bias since they're not stable (like a lot of us, LOL!) using fixed bias and can cause catastrophic destruction of the output circuit. Auto bias can be off +/- 20% as well!
Also, the newer production tubes of the classic Triodes do not have the same quality of the NOS tubes from Western Electric, RCA, Sylvania, etc. These newer tubes are inconsistent and can't hold their bias. This can be a problem with auto-bias, since they can run too cold or too hot. Amps with adjustable bias, that can measure current, can make these new production variations unimportant.
My $0.02,
Pete