Mossberg's got an interesting observation here, but he's missing a really important point: Apple's core profit engine is selling hardware. Amazon and Google make money by selling services and non-hardware goods.
Since Amazon/Google make money regardless of platform, they have an incentive to produce content for Apple's platform. Since Apple only makes a real profit if/when you buy their hardware, they have no incentive to do the same.
From the article:
"...it stems from the different business models of the big rivals. Google, Microsoft and Amazon are primarily software and services companies, though each makes some mobile hardware (Google through its Motorola subsidiary). But Apple, while famous for making good software, sells that software almost entirely through iconic and expensive hardware, from which it makes the vast majority of its money."
I think he covered that the different companies have different business models.
Since Amazon/Google make money regardless of platform, they have an incentive to produce content for Apple's platform. Since Apple only makes a real profit if/when you buy their hardware, they have no incentive to do the same.