Some forgot to renew the domains, some just don't want them anymore or they moved on to other projects.
For most people these Expired Domains don't have any value, because they don't know about the Google age factor or Page Rank.
They just see a bunch of Domain Names someone else deleted and move on, but for the people who know about stuff like SEO or Backlinks, Expired Domains are money just waiting to get picked up from the street.
The only problem is to separate the good ones from the not so good ones. This video show you the easiest step by step to get all of High PR domain.