Why are audio books not available in some countries?

The availability of content in certain territories is decided by the publishers of that content, not the vendor selling the content.

Digital audio and video is no different to certain printings of books being available in the United Kingdom but not North America and vice versa, for example. Sometimes content is released in different times in different countries and sometimes not at all.

I think you might find this helpful. Essentially Audible has chosen not to allow in-app purchases on iOS as they will need to pay a fee to Apple for every purchase. Other content providers like Netflix also choose not to allow in-app subscriptions for this reason.

Apple publishes the country availability of media services at

Availability of Apple Media Services - Apple Support

To ask that they provide more info about "why" , you can try

http://www.apple.com/feedback