I expect it is cost - The fear of pilots coming back to them in 20 years time saying that they've ruined our hearing and we weren't provided with good enough hearing protection. I can't see a cynical argument beyond that. We all know Dave Clarks are comfortable but rubbish. The early noise cancelling headsets made you feel better, but were letting all the damage through. I certainly don't want to be wearing a helmet for 7 hours - Tried one for a few flights and found it OK, but not very comfortable. And I cannot see any way that the noise attenuation would be as good as a modern headset.
I don't see conspiracy theories as helpful. Its probable that the operators just have to provide good quality hearing protection and I'm not particularly surprised if that Peltor headset is the best one they can find.