Thank you for reading, Joe!

I've thought about that -- a great deal, in fact -- and I believe it was a deliberate stylistic choice. I believe Arthur Symons, who was skilled in the art of prosody, had something very specific in mind, that his redundancy of "rain" throughout the whole poem -- including the careless-seeming rhyme you mention -- was in the end not sloppy or careless but intentional and thematic.

I can't prove that, of course. But having read more and more of his poetry, it seems unlikely to me that he did it by accident.