Why Crisis Core is on PSP, not DS

From the interview: "Square Enix is a firm supporter of both handhelds, with a number of great titles available on both Sony PSP and Nintendo DS. However, why was (arguably) their biggest game developed exclusively for PSP, instead of the more-popular DS? Pocket Gamer talked to producer 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."