As an avid buyer of Doctor Who DVDs, that is a good question; more and more region 1 discs are going out-of-print. The pattern is weird too:

William Hartnell:
The Rescue/The Romans
The Web Planet
The Time Meddler
The Gunfighters

Patrick Troughton:
The Invasion
The War Games

Jon Pertwee:
Terror of the Autons
The Time Monster
Planet of Spiders

Peter Davison
The Visitation
Black Orchid
Earthshock
Time-Flight
The Awakening
Frontios
Planet of Fire

Colin Baker
Mark of the Rani
The Two Doctors

Sylvester McCoy
Battlefield
Ghost Light
The Curse of Fenric

And the number of entries fading from print is already growing as we speak.

Some of these make perfect sense to stop printing, like The Web Planet, the Gunfighters, The Time Monster, Planet of Spiders, Black Orchid, Time-Flight, Mark of the Rani, and Battlefield, but others are ridiculous to stop printing, like The Rescue, The Romans, The Time Meddler, The Invasion, The War Games, Terror of the Autons, The Visitation, Earthshock, The Awakening, Planet of Fire, The Two Doctors, Ghost Light, and the Curse of Fenric. Why these faded from print has baffled numerous American and Canadian Whovians, and we can't be sure why. For example, why did The Two Doctors fade from print and not Revelation of the Daleks? Well, besides the fact that Revelation of the Daleks is severely underrated by the British fan community for not reaching British tastes to begin with but sits well with Canadians and Americans (lots of violence, little innuendo; just how they like it), the Two Doctors was also actually one of the earliest released by the DVD company that manufactures Doctor Who dvds for Americans and Canadians. It's normal for DVD companies to gradually stop printing discs that they were printing for many years.

But that still doesn't sufficiently explain some of these releases going out-of-print less than five years after their release. Part of it is probably due to sales; as I implied, some of these faded from print probably because some of them weren't very good. But another common thread to the ones that tend to fade from print is that they are often later in the show's, and the Doctor's, chronology. Earlier entries almost seldom fade from print because fans make the awful mistake of trying to watch the whole series in order, or at least start with the beginning of each Doctor, which, for even the most dedicated, is usually a terrible mistake. Peter Davison, Colin Baker, and Sylvester McCoy all had weak openings. So that explains why The Invasion and The War Games faded from print; chronologically, that's the last of Pat the Trout that's on DVD.

And of course, some of these are on NetFlix. The Visitation and The Curse of Fenric. Why buy those on DVD when you can get them online, and numerous other good movies and shows, all for only a little bit of money a month?

4749367 Tell me about it....Classic Who's Netflix page is TERRIBLE....Only good thing I got from it was that I got to watch Pyramids and Mars.