Sometimes poets [even the good ones] are reduced to using filler lines. They come up with a few really good ones, and then use filler material of lower quality around them.
Bite your tongue! Not POE!
Seriously, at various times, he apparently told different stories about the composition of this poem. To some, he told that he just whipped it out over a couple of hours. Later, though, he wrote an essay about a lengthy construction of the work.