If you look at your website Analytics, you might find that the pages that the search engines are sending the most traffic to are quite old. Here’s some data from our website, for example, showing that of the traffic from Google search, the most popular landing page at the moment is from 2011:
Google loves old stuff, especially if it can measure that people are clicking through to the page and not coming straight back. What’s more, old content will have had time to have picked up links, another one of the major factors in a good Google ranking. However, some companies are scared of old information on their website. Despite its massive marketing value, they don’t want people arriving at the site on a page which talks about products which were current several years ago. So they pull the page on their next replacement website, perhaps redirecting the address to some shiny new equivalent. And sadly, the new page turns out not to have whatever secret sauce made its predecessor so attractive.
The way to overcome the problem is to keep those popular old pages updated. It turns out (according to many SEO experts, anyway) that Google likes updated old pages even better. So make sure you know what assets your website contains, treasure them, and spend time keeping them up to date.