Because Nintendo (like Apple) tries to make their new revisions of hardware the "mandatory" version and tries to obsolete your version.
Sure, there are a bunch of PSPs, but you never feel like you're missing out on anything because you have a 1000 instead of a 3000. Same with PS3s and Xboxes for the most part.
Just how many physical games actually required DSi? None I can think of that have rendered the DS "obsolete," that's for sure.