Why are pilot episodes often unaired?

Question

I frequently watch TV series on DVD and very often I see the "Unaired Pilot" episode advertised (most recently Supernatural).

I understand these are the test episodes which will be given to the networks for them to decide whether or not to pick up the show. However these episodes are often crucial in setting up the character motivations and back story.

Why would they remain unaired until the DVDs? If the network has already paid for the production why confuse viewers?

Answer 1

On the contrary, I don't think that pilot episodes are often unaired. They are usually aired as a show's first episode unless there are issues with the production quality, the cast, or similar.

From Wikipedia:

Most pilots are never publicly screened if they fail to sell a series. If a series eventuates, pilots are usually – but not always – broadcast as the introductory episode of the series.

A pilot episode is generally the first episode of a new show, shown to the heads of the studio to whom it is marketed.

From TVTropes:

Even when a show is picked up and given a timeslot, there is no guarantee that a pilot will ever reach the air. They often do, usually as the premiere.

Reasons why a pilot might be unaired include:

• Low production quality: Some pilots are created on shoestring budgets and might not be able to afford the lavish sets and technology required for prime-time television. They are effectively demos. The pilot for the quiz show, QI, is a representative example.
• Change in cast: The cast used during the pilot might not have been available for the main series. Wikipedia throws up Gilligan's Island and Temple Houston as examples.
• Change in script: The network might have been satisfied with the feasibility of a project, but required changes to the script which necessitated a complete reshoot for the first episode. The pilot for Gilligan's Island serves as an example here too.