Complex sentence: This is probably because it was not clear from Dreben's teachings how philosophical theories could be advanced after Wittgenstein or after Quine--that is, how a philosopher could further human understanding apart from studying the history of people who talked past one another.
Split into simpler sentences: This is probably because it was not clear from Dreben's teachings how philosophical theories could be "advanced" after Wittgenstein or after Quine. Another way of putting the same point is that it is hard to see, from Dreben's lectures, how a philosopher can deepen or further human knowledge and understanding apart from studying the history of people who talked past one another.