If you combine the sales of games released on the Apple portables, Android devices, Palm OS, Windows 7 phones and Blackberry smart-phones (for the vast majority of games) the revenue is too small to support the games with the budget of most Nintendo DS games.
Small developers making tiny games that get unusually success are doing alright on these platforms, but most larger products that are released on these platforms lose a remarkably large amount of money; and publishers are accepting these losses in the hopes that a sustainable market will emerge in the future. The core reason for this is that games seem to only sell particularly well if they're very inexpensive ($2 to $5), and most games struggle to achieve a couple hundred thousand units sold. For the market to become viable for these developers they either need to sell a couple orders of magnitude more games, or they have to maintain the same sales while selling their games for (closer to) $20 to $50.
