Two main reasons. First, in July 2001 the POPIN web
site files were migrated from www.undp.org/popin/ to www.un.org/popin/.
The files were left on the former location for several months, but
were then removed. If you have bookmarks with including "undp",
try removing the "dp". Second, in October 2001, publications
of the Population
were moved from the POPIN web site to the Population
Division web site. The links to these publications on the main pages
of the POPIN web site still work, but bookmarks for the publications
will not work. Look for the publications you are interested in either
on the POPIN site or on the Population Division site.