Michael Mace writes about the reasons:
The first is that the two platforms that had been driving the most app sales, Palm OS and Windows Mobile, are not growing in total.
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The second problem is that as the mix of devices shifts from handhelds to smartphones, I think application downloads become less attractive.
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The third problem is that application sales haven’t tracked the growth of some of the smartphone platforms (in particular Nokia S60, which is producing most of the reported growth in smartphones).
