The reason for this, I think, is that this stance is perceived as threatening commonsense and essentialising understandings of self and agency. It is a realisation which is crucial to SFL as political project, and to critical literacy as educational project. If lived, it puts puts everything up for negotiation, for challenge, and thus for change. It focuses on socially maintained differences and invisibilities, with the aim to make their principles and structures explicit. And it is precisely in making the naturalised invisible visible that SFL's de-marginalising power resides.
Systemic theory is a way of doing things.
Now, our environment and social relations are getting increasingly complex and 'technological' 2 . These differences are therefore becoming increasingly difficult to negotiate, challenge, and change. SFL offers means of talking about and dealing with some of these phenomena, without essentialising any of them by speaking about them in abstractio, and thus placing them beyond scrutiny. SFL's descriptions offer explicit ways into these complex and technological meaning making practices. I read this as saying that 'social relations and social differentiation' are somehow too indeterminate, or perhaps too self-evident, to enable or need scrutiny.