Hello. I can't say why yours wasn't selected(as mine wasn't either) but I can at least give some feedback on how to improve your piece I think.
Perspective - Parts of the image look like its a fish-eye camera lens(clouds, mountain), but other parts aren't really following this. Such as the ground itself curves a little bit upwards on the right, but that's it. It really just looks like there is a hill behind the characters over there. So the sky/mountain are in one perspective but the ground is in another.
Anatomy - There are a few little issues on the main character though he is mostly good. His far eye is one. I think it would be more hidden by his nose. Also his stomach/chest/far shoulder all look pretty ambiguous. The torso is especially blending with the closest arm. Even if that would be the case in this lighting, you as an illustrator have a duty to bend reality so that your image is more readable to the viewer.
The second character's arm looks very rushed. Compared to the main guys hand(close to snail) which looks Really nice, it just makes the secondary dude look worse. Sadly your picture is only as strong as its weakest element in many cases.
Those are the main things that stick out to me. Overall the composition and colors are working pretty well. The colorful texture on the snails back looks especially nice.
Hope that helps. Cheers!