Long sales pages are able to present the pain the reader feels, detail the solution they need, provide an offer, establish their authority, highlight customer success stories, counter objections, and reverse risk. (This is all yanked from Sean D'Souza's book, the Brain Audit: http://www.psychotactics.com/the-brain-audit-marketing-strat...)

It would be hard to do all of this with a short sales page.