Now, I want some real answers - no "because Nintendo sux!" or "because gamerz moved on from the portable ghetto". It's something that's puzzled me for a while.
I'd say 2008 was the peak of the influx of DS support from 3rd parties. During this time it was almost inconceivable to even imagine the PSP turnaround that the system has experienced in these past couple years. When 2009 came around I started to notice more and more 3rd parties announcing games for the PSP. Most were ports, but then they started to announce new, big production games, and many of the teams that developed DS games before were now moving on to the PSP (Image Epoch, for example - originally a DS-exclusive developer, now they've made more PSP games than any other system). 3rd party support continued to expand and to this day it's still growing.
Now the PSP has greater 3rd party support than the DS ever had in its entire lifespan. It's honestly getting PS2-level support from Japanese developers. I don't think the game industry has ever seen such a turnaround. This PSP shift hasn't just left the DS in the cold, but also its successor, the 3DS. This is manifested not only though weekly sales but also in weekly game announcements - the PSP gets more announcements in Japan than any other system currently. While the 3DS sees a significant surplus of shipments in Japan, the PSP is constantly suffering shortages. I honestly don't know what Nintendo can do to reclaim their former position, aside from maybe paying Capcom for MH exclusivity (even that's a stretch) - which I honestly don't see them doing. At the same time, I have no idea how Nintendo lost their position to begin with.
Did Monster Hunter singlehandedly help PSP rise to the top in Japan? Is it just because certain developers never gave up on the PSP, even when sales were somewhat bleak many years ago? What caused the PSP to cleanly swoop the lead position from the DS in Japan? I honestly can't recall a shift in tides as significant as this.
Again, SERIOUS answers only from SERIOUS gamers like myself. seriously.