I have no specific information on the humanities side aside from anecdotes and personal experience, so I'd be interested if you could elaborate. Is it perhaps possible that most of the students that are
accepted are fully funded, but that there are far fewer positions and so de facto there are far fewer opportunities for humanities students? Also, my specific example of a friend in Eastern European Political Economic Thought was at an Ivy League school, where he has a tuition waiver but no stipend (so one could call that "fully funded, but unpaid").
Just to elaborate on my limited view of the funding landscape, in STEM disciplines I can name 5-10 major public funding organizations easily (NSF, DARPA, DOE, ONR, AFOSR, NASA, ARL, DHS off the top of my head). My uninformed guess is that there are not nearly as many organizations for humanities, and that they are not nearly as well funded, but that's just based on my hypothesized motivations for the funding (military industrial complex, communists/terrorists/whateverists etc.).
If you have any light to shed on the subject, I'd be interested :-)