Well its a little daunting putting up an answer when Apollo has assumed the role of professor in chief assignment marker...but...
My guess from my most basic knowledge would be that its (obviously) cartridge specific & prob got something to do with the lede/lead (not sure of spelling--pronounced "leed"--the kinda free-bore area forward of the neck), as i underatand that is effectively part of the combustion chamber & effectively part of the cartridge design.
I guess the guys making/designing/doing the r&d check that stuff & go with what they deem is the most efficient for the said cartridge brief.
So a cartridge with a smaller/shorter lead volume can get away with a shorter neck to hold the bullet as part of its initial combustion chamber is forward of the case.
If it had a longer neck it would block this area off & increase pressures...hence trimming/max length of brass for any given cartridge...??? ?...
Ah hell i dunno
....
Thats my guess...
I may make no sense to anyone, but make perfect non- ballistician sense to myself...
The man who knows everything, doesnt really know everything...he's just stopped learning...