It comes from about 1994 when the internet was (relatively) new, exciting and unknown. There was a perception that people wouldn't always assume that there was any more content than they could immediately see, despite all similarly text-based applications having vertical scroll and the clear visual clues given by text that clearly hasn't finished at the end of the first screenful.
But nearly 15 years ago, that was debunked as studies were showing that people were increasingly comfortable with scrolling - see Nielsen's reports from [1997 and [url=http://www.useit.com/alertbox/scrolling-attention.html]2010](http://www.useit.com/alertbox/9712a.html). (Nielsen is pretty much the usability guru, and has a very high reputation in his field)
A key aspect as to whether people will scroll is whether the page encourages them to. If the page obviously continues off the bottom of the first screenful, there's a good incentive to follow it down. If there's a big white break that coincides with the bottom of the screen then they might well think that's the end. The trick there is to check your page out at common screen sizes and just make sure that there's always some visual clue that the content continues beyond.
Yes, you want to have your hot content near the top, where it's going to attract people's attention ... but don't think that you have to keep everything on the first screenful.