[HowardC]

Well short stories are rarely long enough to fill out a whole film so they are expanded upon, which means deviaton from the original story, which means leeway for sequels.

Novels on the other hand have to be condensed to fit on a single film, and studios rarely allow sequels to be made before the first film proves itself. Thus even if the film does well, you've already told the story and thus no room for sequels.

Also keep in mind that horror movies don't count. Because they are usually cheap to make and don't require star power, if one is successful they make sequels until it stops making money, regardless of the source material.