The majority of people on the internet use normal 28.8 or slower modem connections. If you have a page which requires over a minute to download on this kind of connection you've almost guaranteed that your visitors will go somewhere else. Search engine spiders do not like long documents. Many of them will stop after 100kb or so - it's anyone's guess if the spider actually looks at a page that is a megabyte in length. They will scroll if they read something of interest, but they will not scroll very far.
Most people tend to prefer clicking on links to scrolling down the page.
Navigation on a very long page becomes difficult, if not impossible. One of the major advantages to splitting up a long document into many smaller pages is the control you gain over navigation. Other web sites will find it difficult to link to specific content on your page. This is the web, and webmasters want to link to pages on a site which pertain to their own content. If you have one long page, you effectively discourage this kind of linking, which means you will get fewer links.