Well, since nothing Nirvana did was anything original, all I can say is that he was successful at getting people to swallow a lot of recycled punk. It's not that he wasn't "that good of a songwriter," it's that he wasn't a songwriter at all. He was a pathetic, unwashed, unintelligent plagiarist. Wierd Al Yankovic's parody of "Smells Like Teen Spirit" was more imaginative than the Nirvana's "original" version.
I can't believe that I'm saying this, but even Metallica were more original than Nirvana (and Metallica had very little originality at all, especially after Dave Mustaine left/was fired).