Halliday & Matthiessen (1999: 543-4):
The reason is to be found in the grammar of the textual metafunction. In order to function with the requisite value in the message, which means either as Theme or as focus of information, they cannot remain as complete clauses; they have to be "packaged" into single elements of clause structure, and the only available constituent for this purpose is the nominal group. … It can then take on a clearly defined status in the grammatical construction of the discourse.