I think that is, in part, due to a few particular points:
1) In this, only the second story of his mature period, he is already introducing many of the major themes he would deal without throughout his career.
2) It is, in some ways, a "dry run" for the final section of "The Call of Cthulhu", with all the implications the two share.
3) His development of atmosphere here is, in some ways, more concentrated because of its brevity; though such a short tale, it nonetheless contains a considerable amount of the horrific (or terrific, if you prefer) content of his later, more developed work.
4) It is a powerfully dream-like story, where he is already blurring the lines between reality and dream so that the reader is never entirely certain which is which.
These are a few of the points which come to mind. There are others, I think, but this may help give an indication of why, despite its flaws, this tale nonetheless continues to raise such interest and respect...