This appears to be a downsized image, because when I zoom in on it to inspect more carefully, it's very pixelated; therefore, no opinion as to its suitability for Stock can be formed. In particular, focus and noise can't be evaluated. If you want useful opinions as to why it was rejected, please upload in full size.
Why don't you submit full size images? Doing so increases your chances of a sale. If you zoom in on this image, you'll see how quickly it becomes pixelated.
Additionally, the image has a White Balance issue - too blue as indicated above by @Nancy OShea
There is always a reason. For example "quality issues" which tell us that it doesn't meet their quality standards. Please read this "standard wording" for us.