Why does postmodernism (via discourse theory and deconstructionism) criticize interpretivist studies of culture?
This is a good question. A good place to start is it look at modernism and postmodern. Modernism is largely a belief in objective reality. Through observation, modernism believes that we can get a truth. Postmodernism call this into question. They say that we know "truth" through the experience and perspective of the "knower." This basically means that truth is relative. This is why what is true in one area of the world is not necessary true in another. To put it another way, there is no one meta-narrative that can be interpreted and that conjoins everything.
Foucault talks about the archaeology of knowledge. What he means by this is that there are different streams and traditions of knowledge and they all have interpretive validity, because all of this discourse is relative. In his book, Power and Knowledge, he writes:
"Each society has its régime of truth, its ‘general politics’ of truth: that is, the types of discourses which it accepts and makes function as truth; the mechanisms and instances which enable one to distinguish true and false statements, the means by which each is sanctioned; the techniques and procedures accorded value in the acquisition of truth; the status of those who are charged with saying what counts as true."
From this quote, you can tell that if anyone or any culture claims to have the truth, this discourse is an exertion of power and not an interpretation of truth. In other words, all cultures and interpretation of culture are relative. This is why postmodernism and deconstructionism call into question the interpretivist view and all interpretations that claim too much.