That's probably because of the target marketing audience that Sony and Microsoft- contrary to Nintendo of America's audience- have already catered to: RPG fanatics.
NOA, to me, doesn't put enough emphasis on RPGs even though they would sell on any console like hotcakes. I don't know what their stubbornness on importing RPGs is about. Sure there's The Last Story, new Dragon Quest games, and Xenogears, but if you visit their official site, the coverage they give on Japanese greats that would do well here is lacking.
My other gripe with them is they also don't give Etrian Odyssey 4 for 3DS (already a hit in Japan) the time of day here. You have to watch the Youtube promo
-still in Japanese- just to know the EO series is still going strong!
I agree, these RPGs would extend the life of both DSes. If you noticed that about PS2 near its end, the last games released for it were mostly JRPGs, That's how the PS2 shelf life lasted a few extra years, so this theory is tried and true.