Hideki Imaizumi and executive producer Yoshinori Kitase about exactly that: 
"Yes, there are a lot of DSs out there - but does that mean that our products sell in relation to that? That our sales grow with how many DSs or PSPs there are out there? It's not really the case."The PSP seemed appropriate for a number a reasons, one being the more adult age group the PSP seems to attract: "the demographic was higher teens to young adults as opposed to small children. The PSP seemed closer to that demographic than DS in general. So we feel we really made the right choice."
As a joke, he added "We'll make a DS game called Brain Age: Final Fantasy for DS [laughs]." To be honest, we wouldn't mind trying that out.
