In Japan, portable sales are generally linked to the weekly releases. There was a big initial rush when the 3DS debuted and now that the rush is over, consumers are more likely to be swayed by new software releases than just the prospect of buying a new handheld.
Sales will fluctuate based on new releases and currently, there really isn't anything major being released for the 3DS.
Until developers shift their major IPs over to the 3DS and taper off DS support, other than the prospect of having the latest handheld, there really isn't a compelling reason to jump on a new platform until the games a consumer wants to play are only available on that latest platform.
By the same token, when soft support tapers off further on the PSP as development shifts to the NGP, watch sales taper off accordingly.
It's not exactly complicated the way this works.
