>Up until early 20th century philosophers had material contributions to make to the physical sciences
I think what changed was physics became much harder to understand. Pre Einstein it was fairly easy for someone without much physics training to grasp Newtonian stuff. With the advent of general relativity and quantum mechanics it became hard to understand without years of study of physics which most philosophers do not have which led to their work being not very useful to the physicists. However philosophical insights by people who do understand the physics can be good in my opinion.
And on that point, there are philosophers who do get the physics, and who are making material contributions. However they are significantly marginalized by their predecessors and often must directly compete with those who do not understand that physics for grant money, university resources and students. They are an unheard minority, quite unfortunately.
I think what changed was physics became much harder to understand. Pre Einstein it was fairly easy for someone without much physics training to grasp Newtonian stuff. With the advent of general relativity and quantum mechanics it became hard to understand without years of study of physics which most philosophers do not have which led to their work being not very useful to the physicists. However philosophical insights by people who do understand the physics can be good in my opinion.
Feynman seemed good, for example. Simple stuff
http://lesswrong.com/lw/99c/transcript_richard_feynman_on_wh...
a bit more subtle, philosophical at the beginning:
http://www.feynmanlectures.caltech.edu/II_28.html
Feynman and philosophy:
https://philosophynow.org/issues/59/Richard_Feynman_Accident...