I won't argue that, but there's also the question of where the market is the most heavily saturated. In this case, the DS is receiving a great deal of RPGs. Thus, it's safe to assume that the mainstay of the RPG handheld market exists on the DS at this point.
I understand a game being the big fish in the small pond can be a good idea, but sometimes, even with all of those other fish in a large pond, there's enough food to go around to sustain all those fish and then some.
Also, I think FF carries with it enough of a brand name that they'd be successful on any console, but I think they'd get more on the DS because the DS represents the audience which is the least likely to have played their games before.