The second, perhaps more common, reason is that the authors simply do not clearly state the significance of their findings. Worse, they are completely unaware that they are not doing so. Most papers are written in a dry style, listing experiments in the exact sequence that they were done, with no effort made to engage the audience. The result is that the reader is slammed with a series of complicated experiments with no end in sight and fails to see the significance. It is a classic case of being lost in the trees and not seeing the forest, and I have seen it so many times its not funny. The Reviewer/Editor cannot see it because the writer is not stating it clearly! One obvious reason for this is that scientists get almost no training in writing. But I feel there is also a cultural problem that scientists don't think that it is important to write clearly. While they may not directly say this, few make any effort to learn how to write clearly. Whatever one thinks about writing is up to them, but having been on many study sections and reviewed numerous papers I am absolutely certain that clear writers are getting most of the prizes.
So what can one do? Besides reading and re-reading the book on writing clearly ("On Writing Well" by William Zinsser) until the cows come home, one thing that helps me is to write the abstract first. I know that this is exactly the opposite of what the pundits preach, but I always write and re-write the abstract first (often languishing in it for weeks or months), until the sequence of events (and significance) is clear in my head. Sometimes this reveals major insights, and at other times it tells me that the Science is solid but its not that significant after all. Once this logic is clear, the paper can be written. Often this exercise also helps in framing/designing final follow up experiments, and that is a bonus. So happy thinking and happy writing!