That and more: Apple's platform probably won't be adopted by anyone else, so even though the user base is attractive right now, down the road it would look diminutive compared with the user base for an open platform that could gain support of multiple vendors.

7-8 years ago I used to develop mobile apps in both Palm OS (closed platform, just one vendor) and J2ME (open, widely adopted). Palm had the more attractive installed base back then, but as their fortunes changed, that investment is now worthless. Compare with J2ME, which kept growing thanks to support by most vendors.