Why is the use of archaic English words considered “bad poetry”?
It’s not “bad poetry”, it’s just precious and therefore unnatural, especially if the poem is written in archaic language throughout. Then it will appear almost as a parody. But I think that one archaic word in a poem, for effect, can be acceptable. It has more to do with language usage than with poetry itself. Edit: I just want to add that what I don’t like about such poetry either is that very often, people don’t really know archaic grammar and vocabulary and write sentences that are not correct. So it does not sound authentic. But maybe archaism is no longer accepted in our time. Edmund Spenser used archaic words, but I think that in his days, people did not have the same perception of what “archaism” was.