Well in terms of the DS, i found this little exerpt.
'Pokemon games are incredibly successful on the Nintendo DS. Until now, there have been more than 186 million units sold in the series and the recent release of Platinum is set make that number even bigger. Junichi Masuda, who has for a long time worked on the Nintendo Pokemon series, believes that the main reason behind its appeal is the accessibility and the flavor present in 2D game experiences.'
Link for a further read: