Why The Residue Is Not Labelled 'Proposition'

Halliday & Matthiessen (2014: 143):

The remainder of the clause we shall call the Residue. It has sometimes been labelled ‘Proposition’, but this term is also not very appropriate; partly because, as has been mentioned, the concept of proposition applies only to the exchange of information, not to the exchange of goods-&-services, and partly because, even in the exchange of information, if anything it is the Mood element that embodies the proposition rather than the remainder of the clause.