I've run into the "why don't they release complete seasons of the classic series" question a few times. Here's why they don't.
First, of course, none of the seasons from 1963 to 1969 exist in complete form anymore, with many episodes missing from the archives. And while we technically have all the 1970-74 era episodes to view, a number of them require extensive restoration/colorization work to be considered releasable on DVD. (You won't see classic Who on Blu-Ray until they fix the format to be more friendly towards old videotaped productions).
But the main reason is that BBC Video/2 Entertain have decided to make Doctor Who the most thoroughly documented TV series release in history. With only a handful of exceptions every story (and of course each story consists of anywhere from 2 to 14 episodes) has commentary, a behind the scenes documentary/retrospective, video featurettes, DVD-ROM content, and other stuff. There have even been a few releases that include "TOS Remastered"-style upgraded versions (though unlike the original releases of Remastered, viewers have always been given the choice of watching one version or the other, or else the upgraded versions have been included separately as movies). Right now BBC Video is in the process of reissuing some of its earlier releases to include additional featurettes. This is a far cry from most TV series releases, especially vintage shows, that you might be lucky to have a couple of talking head interviews and commentaries. The original Mission Impossible was released without a single special feature included on any of the sets.
The tradition is continuing with the revived series, as each individual episode is accompanied by a 45-minute Doctor Who Confidential documentary, though thus far only a handful of full-length DWCs have been issued to DVD or Blu-Ray, primarily because they'd have to double the number of discs in the set to include the whole thing (ignoring music and footage licensing issues for the moment).
Personally, I think it's a great way to release the classic series. And it allows us to pick and choose the stories we want. If you really don't want The Twin Dilemma clogging up your collection, you can ignore it. And in the rare case where a series of connected stories occurred, such as The Key to Time, they've been kind enough to put them out as box sets (though North American releases are sometimes issued separately, such as the recent Kamelion Tales set).
Last year a member of the Doctor Who Restoration Team was quoted by the Canadian Doctor Who Information Network as saying BBC Video plans to continue releasing Classic series stories in DVD until at least November 2013. My assumption is, for those with deep pockets, someone like Amazon is going to offer a "motherlode set" of everything around that time (there was in fact a "mini-motherlode" set issued a couple years ago, IIRC). And of course there's always the hope a new codec or something will come along that'll allow HD-format issues of Classic Who (and Trek TNG, etc) without the need to make wholesale revisions.
Alex
