I wondered this myself. Of course, the matter is more complicated than what I'm about to say, but I think one factor that lead to the popularity of materialism is the success of the empirical sciences. Some materialist thinkers and scientists like to appeal to science as a way to argue for materialism. For example, Paul Churchland argues that the empirical sciences have shown us that we were wrong on several issues that we took for granted and "commonsensical"  (i.e. geocentrism) and that one day when the neurosciences are developed sufficiently then those neurosciences will prove us that there is no such thing as the commonsensical mind or "folk psychology" as he calls it. Ullin Place also appealed to the empirical sciences. He thought that we can show that "consciousness is a brain process," which he thought that this was a reasonable scientific hypothesis.  

@iwpoe
Hmmm, sounds like rehashed logical positivism.