Peter explains it quite well (no matter how much I disagree that sandboxed Java applets are the issue, and it's Java applications, platform and IDEs which make it powerful):

Most platforms have a single development environment: Android, iPhone and webOS all have pretty much a single way of developing apps. Nokia have two (and that's already more complicated than is ideal from selling the device to new developers): Web Runtime and Qt.

Options increase the barrier to entry - but (and this is a point I made very strongly) so do crap development tools :-/