I love T17 but its So Rare and extreamly hard to collect for and im all for useing old fixtures as my daily in my shop but there is not a single
chance i could do this, i cant even find a lamp let alone a fixture,
so Why is it that there so Rare, were they jsut not popular like T12?

T17 was made invented just Before WW2  lot of those
lighting were use for goverment war effort.  and not
many T17 unif made. until Later VHO T12 came out
left T17 in the dust. but T17 PowerGroove was still
popular at that time.

over years T17 fixture get scrapped.  abandon
then Demand for parts for T17 system dropped
no longer manufacture parts.  so thaf why you see
ballast and starters on ebay. but cant find lamp holders
and starter sockets.  then some times in 90s sylvania
ended production of T17 lamps. then philips stop making them just before 90s hit but made T12 for F90
system bit longer. GE made them until 2012.  the demand for them drop so low that prolly only make less than 10,000 lamps a year. at that point why keep producing lamps at such low demands.  when they stop producing Lamps dried up with in a year.  and people bought them up to keep there T17 going.  now it prolly
will soon be some T17 fixture dumping again this time around due not getting lamps for them.  so just keep on
look out.

What is the advantage of VHO T12 over T17?

The only Advanage is manufacture can Use Existing T12 machinery and useing Beefy cathodes to run the lamp.  at same time rapid start was coming out. 
It use in places on very high celings and  at same time
when you need lot light output.  at one time it was use
for gas station lighting.  and early street lighting.

What is the advantage of VHO T12 over T17?

Newer phosphors can work well with higher intensity, so no need for that large diameter.
Less surface to coat, so less phosphor cost.
Plus thinner arc allows shorter path for the UV so less chance to be absorbed by the mercury, so higher efficacy.
The tendency to boost the efficacy by making the lamps thinner is nothing new in the late 90's, it happened all the time and all the time it had its limit on what the available phosphor could handle.

你好像还忽略了F54 T17，请问它是什么时候出现的呢？