For paid applications, there might be issues with the payment itself.
- Different exchange rate could make it troublesome for developers to do accounting of their income.
- Tax rules about selling things abroad. Does selling software online got categorized as foreign export ?
But that doesn't explain the unavailability of free apps.
Maybe they think of providing support but don’t want to deal with the various languages of the world.
Most developers select the all regions box but some developers have their own reasons for wanting to only display their application in one country
And the applications won't automatically be made available to the new Mango marketplace. Developers who has abandoned their apps wouldn't bother setting the all region availability again.
At least in my experience, that's the case for some applications like Twitter, Flickr, Gowalla