Halliday & Matthiessen (2014: 381):
So the principle which puts the Theme first in the clause is the same as that which puts the Deictic first in the nominal group: start by locating the Thing in relation to the here–&–now — in the space–time context of the ongoing speech event. From there we proceed to elements which have successively less identifying potential — which, by the same token, are increasingly permanent as attributes.