Why is the speaker a skeleton?
To me, the speaker in this poem is a skeleton because he is supposed to be representing people who have been dead for a long time. This is because this is a poem about how people never change.
The major theme of the poem is the idea that people in modern times are just as warlike and violent and awful as people always have been. So it makes sense to have the speaker be a skeleton that has been dead a long time. This speaker can sort of "wake up" and say "wow, how come things haven't changed in all the centuries since I died."
So having a skeleton narrate helps to make the point that the poet is trying to make.
The speaker of the poem is a skeleton who has been long buried in the local churchyard. The noise of guns, being fired out at sea in distant target practice, has awakened him, along with all the other skeletons in the cemetery. This is a play on the fanciful idea that a noise can be so loud that it can awaken the dead. The dead man's voice in so many poems is in stark contrast to the human perspective of the subject of immediacy and temporalness. In this situation the skeleton represents the fact that war as a brutal human convention will continue forever, a bleak assessment of man's inability to confront its abilities and desires successfully.