[jamesu]

I'd attribute it mainly to the APIs available. Right out of the box you can easily loads PDFs, images, and make "Document" oriented applications. You can also do rich text editing and even embed web content to do more advanced layout of content. e.g. If I was doing some sort of "knowledgebase" app these sorts of things would be indispensable.

But also there's the market to consider. Macs users seem to like to sink their money into nice looking native apps which organize things.