Basically, it's because it felt like enough.
If I read a story's first chapter and it bores me out of my mind, that's enough to drop the novel. The rest of the story doesn't matter, I was bored out of my mind from the very first chapter after all. That much is plenty.
So, if I wanted to leave a review on why I dropped the novel, I'd do it with one chapter because... Well, that's all that I read, really.
First 10min of any media are the most important ones, folks. Its from it that you hook your customers... Fail to hook them in 10min, you failed at the most important portion of your product.
For most people, if they aren't hooked quickly, they'll just leave the story and move on... Some other people will leave ratings, and some will rate and review... It happens. At the end of the day, all those people just felt disappointed by the story after the very first chapter, so they dropped it~