This poem features a giant lion's paw pressing down on someone like a huge boulder, so we'd forgive you if you thought the setting was a Voltron cartoon. With that imaginative bit of personification aside, though, most of what happens in "Carrion Comfort" involves the speaker speaking to…himself.
In that way, the setting's probably best understood as a personal, internal one. After all, the speaker has just come through what sounds like a pretty intense bout of despair, the depths of which can only really be found in his own mind. By the same token, the speaker is questioning God about why he's had to go through such a prolonged trip to Bummersville. That kind of spiritual questioning—even if it's spoken out loud—is the kind of stuff reserved for an internal struggle. So think of this poem as a kind of early version of Inside Out.