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

+1 for Sylvania Chrome domes , the Goldilocks of tubes , not too little, not too much .

How many tube guitar amps are out there? Small niche? I'd bet they're 90% of the tube market...gotta scratch that niche.

Yes but current tube production is not critical to the communications business, like in the past. Tube makers could do better but where is the motivation.  Product longevity does not keep the line moving.  

"gotta scratch that niche" ..nice
It seems as if more and more tube products are now out there compared to 15-20 years ago so,
while still not huge in the market I do think it is growing
.AND,
as Wolf_Garcia  stated
"how many tube guitar amps are out there?".
quite alot I'd imagine.
So maybe there would be some incentive to MFG some quality Tubes eventually.

Here is stuff I have that needs Tubes;
Stereo-
Schiit Audio Freya
Primaluna Prologue 5

Geetar Related-
Dr Z Rt 66 Head
Egnator Rebel 20 Head
Egnator Tweaker Head
Peavey JSX Mini Colossal  Combo
Peavey 6505 Piranha  Head
Gibson/Kalamazoo Model 1 Combo
JetCity JCA20 Combo Combo
Blackstar HT1 Head
Orange Micro Terror Head
Orange Micro Dark Terror Head
Vox AC4T1 head and combo
Epiphone Valve JR 5 watt head
VHT Special 6 head [2 of them]
Blackheart Handsome Devil 15 watt head
Blackheart Little Giant 5 watt head
Blackheart Killer Ant 1 watt heads[3 of those!,2 modified]


So,that's around 22 things around the house that use Tubes.
A few are only using tubes in the preamp section and
 all the others are all tube
so I am all for increased MFG of quality tubes