With 2-column pages, one column tends to contain many distractions and exit points from the Buy Button, affiliate link, or Adsense. This is fine for the content pages where people can navigate around your site to "learn more" and develop their trust in you, but the sales letter page works best in one column.

For extreme examples, look at lead capture pages with the big arrow pointing to the subscribe form and any other links being hardly visible on the page.