From the FAQ:
What's it going to take to get OZ in the stores, in every retail outlet on earth. I'm sick of waiting for it to be released. It seems that there will be another ICE AGE before this happens!
It is released. Ask your record store to stock it.
So I guess the answer would be, that no one has requested that your favorite distributor stock it. Or alternately your distributor didn't order any from Barfko-Swill. Now apply to all releases in question, with the exception of the limited editions probably.
With Amazon, they have affiliate sellers who list things on the website that Amazon itself sometimes doesn't stock. I'm not sure why Amazon doesn't stock them, that is a question that I have not seen an official answer for. It may have something to do with Music Today being a competitor in some way, but that would be purely a guess.
So I guess the best chance of getting it into your local store, if you still have a local record store (I don't,) would be to walk up to one of the friendly service people and tell them (demand with great conviction and clear diction) that you want them to stock every single Zappa release in the world. Flash 'em money and say "See here, now this could be yours, but you don't carry the album I want so suck it buddy." and walk out the door. I give it a 50/50 shot that the next time you walk in they will have what you want.