David is quite correct. If a studio releases a particular series to DVD and it's not selling well, that studio may still have faith in that show to still sell at some future point down the line. Just look at Everwood. When Season 1 was released, it obviously didn't sell too well. But, the studio had faith in the series and it eventually saw the release of Season 2, much to the delight of the fans of the show. Not only that, but studios are also willing to stagger releases of low sellers with that of more successful releases. If a studio has sold a lot of copies of several shows that have sold well, they may opt to stagger releases of previous shows that haven't sold well, in an effort to please their own consumers. Sometimes, waiting a period of time may be enough to also jump-start sales of the earlier DVD set releases for that title they have released.