It's not a case of unprofessional work the work is very highend.
Observations: you're not choosing illustration - digital as a category. Illustration is a big general term that might be helped by being more specific in categories.
I see a lot of dark images, featuring heavy black. I have noticed my own work sells less when it's dark. Take a page from Yuri's book. He's a photographer not an illustrator but he tends to over-expose to keep things bright bright bright. That may be part of his appeal.
Lastly, there are two kinds of buyer. The first is the graphics pro who is looking for elements, bits and pieces to use to put together their own designs and ideas. Then there's everyone else, looking for complete works that can be placed in a blog or flyer etc. etc. as is, ready to go. Your stuff is very complete. Everything I saw seems ready to place as is. That may limit the appeal for those looking for elements for their own designs.
Just my guesses. Take em, don't take em. whatever. Good luck.