Because it is a test. One would imagine they want to limit the scope initially and expand out as time (and gravitation towards being a full fledged product) allows.
This doesn't seem particularly shocking. Shadow just recently (I believe) opened an east coast data center which now allows for their streaming service to work on the east coast, so perhaps Google is only allotting their US data centers to support this (one would assume initially and that if it is a hit/worth continuing, it'd expand out). Chrome is their browser so either: a) they're wanting to keep support requests limited to things within their control (or at least, reduction of variables is important in the phase they're going). b) they're wanting to be a full fledged gaming platform so they're not interested in supporting others.