Because you have to understand how brand works in the market. DS was the top brand and then it over saturated the market. PSP at that same time gets a new version a hit game and all of a sudden the mainstream things this is the new "DS" because they need something else. Thus brand switches over to PSP. Now obviously it's not as huge as I just made it out to be because DS still sells a lot on average but that's how it works. If the DS hadn't over saturated the market then it'd probably still be on top.
What Nintendo needs to do is release another new model like the DS Lite and give it advertising props and I'm sure the DS will regain its huge brand appeal because it still tops PSP in software and decent hardware numbers showing its not dead.
But if ya really must ask that is why. Just some very bad timing for DS and very good timing for PSP.
