Bamboleo on 25 March 2010
I believe that what started to "kill" the PSP was Brain Trainning and Nintendogs. Those were 2 unique pieces of software only available on the Nintendo system and could not be mimicked on the PSP for the lack of touchscreen.
While this unique games were doing a splash on the charts, PSP was pretty much relying on PS2 ports or games very similar to those available on a PS2.
Then the arrival of New Super Mario Bros and DS Lite was pretty much the final nail in the PSP coffin that set Nintendo as the leader for this gen in the portable market.
