There are also legal ramifications when doing business in a new country.
They (Google, in this case) have to ensure that they are complying with appropriate company/tax/accounting/privacy/etc. laws for each country. This may not be as easy as rubber-stamping some forms, so they have to prioritise countries they are familiar with and are can already do business in.
Google hasn't even managed to accept Canadian developers into the Android marketplace, despite having multiple operations there - they either aren't trying very hard, or they are just really, astonishingly bad at this stuff.
They (Google, in this case) have to ensure that they are complying with appropriate company/tax/accounting/privacy/etc. laws for each country. This may not be as easy as rubber-stamping some forms, so they have to prioritise countries they are familiar with and are can already do business in.