There are many reasons for this to happen, but usually has to do with the company that owns the movie going out of business. But even many movies from companies that are still in business go out of print, just because the company doesn't think they would be able to sell enough copies to justify a run of DVDs.
As time passes, more and more companies are getting websites and putting things on there, but there are still a lot of companies that aren't yet up to speed on the internet revolution.
But this problem is not unique to porn, or even movies. I'm a big music collector and I have tastes that tend towards rare and obscure records, so there are tons of albums out there that I want, that are out of print and likely will be for a long time, if not forever. The only chance I have is finding them used somewhere.
And that's the kind of thing you'll have to do, if you want to see a particular flick. In fact we have some forums right here on ADT that allow people to trade DVDs and buy used DVDs.
ADT's US DVD Trading Forum
ADT's International DVD Trading Forum
There are also websites that specialize in this same sort of thing, like adultdvdmarketplace.com
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