Old and in the way

Reading about the Athena botnet at http://blogs.mcafee.com/mcafee-labs/athena-botnet-shows-windows-xp-still-widely-used got me thinking - if I wanted to create a general attack, I'd go after the oldest operating system I could.  Why go after the brand new machines, with the latest protections?  Why not go after an old operating system, with no internal protection?  Many of these computers might have no protection at all!