SQL2005 is aging well
Because SQL 2005 was the first version of SQL to come in an x64 edition, there are a lot of 2005 installations that customers have no particular need to upgrade - 2000 to 2005 brought huge benefits; 2005 to even 2008 R2 is a fairly small step, which tends to provoke thoughts of "If it ain't broke, don't fix it".