According to the wikipedia entry on his book it represents his turning away from phenomenology to structuralism. What exactly does this mean? How should one understand his book as a demonstration of structuralism? Is it because his analysis is hinged on institutional disciplinary forms?
The sentence in the Wikipedia article prior to the one you cited gives a rough idea of how Foucault moves towards to structuralism.
it is an examination of the evolving meaning of madness in European culture, law, politics, philosophy and medicine from the Middle Ages to the end of the eighteenth century, and a critique of historical method and the idea of history.
Foucault has clearly moved away from a phenomenological perspective if he now concerns himself with the meaning of a sign or a fact such as madness, and how that sign is defined by cultural and political agendas, rather than possessing an objective definition. Concurrently Foucault addresses the impossibility of an objective definition for such a term that is not intrinsically biased.