When it comes to teaching, nobody likes revamping lecture notes and reforming a curriculum. This is especially true when one is oneself not really conversant in the new material. While I think a Economics PhD should be able to teach almost any undergraduate Economics class, one is still drawn to the path of least resistance and teach only what one knows, even when this is outdated. One consequence of this is that undergraduates get to learn what the profession discredited sometimes decades ago. Nowhere is that more true than in Macroeconomics, which went through a transformation in 1970's and 1980's that to a large extend shelved IS/LM, yet the latter is still the core of undergraduate teaching. The fact that those teaching this today were taught IS/LM is the prime reason, and the textbook writers accommodate this.
Some have called current macroeconomic theory wrong with the current crisis and thus there would be the need to a change in research paradigm and thus also teaching. I am not sure about this claim, I would rather call macroeconomic research before the crisis incomplete rather than wrong. As to the teaching reform, that will take ages. One way to the someway fix the broken IS/LM model to make it more amenable to current events, like Peter Bofinger tries by introducing involuntary unemployment that does not necessarily come from wage rigidity. There have been other such attempts, but frankly, they just make the model even less believable and impossible to teach. The true reform should be to drop IS/LM entirely from the undergraduate classroom, except for History of Economics classes.