Some people confuse concept art with illustration. There are major differences between the two. Concept art is part of the development process. Illustration is the final product - a polished piece of work that will be seen by an audience and is therefore intended to be attractive.

Good concept art does not need to have a strong composition, great lighting and colors like a good illustration should have. The main purpose of concept art is to effectively communicate the core of a specific idea, without doing time consuming polish that is typically found in illustration. In practice, this often means using less time on things like composition, lighting, and colors, and focusing on visual structure and how shapes and forms relate to each other.

Furthermore, good concept art is rendered with sufficient accuracy and detail, whereas good illustration does not need to be. If the rendering style of a concept is too loose or rough, it may fail to communicate the core idea well enough. In fact, the more loose the rendering, the less effective the concept becomes at communicating its core idea because the looseness can be interpreted in different ways. So those who will be working from the concept art (like a 3D artist, prop or level designer) can end up creating something that is quite different from what the concept artist imagined but communicated inadequatley. Or, they may have difficulty filling in the gaps due to lack of detail in the concept, and end up using their own time to do concept artist's job. (On the other hand, too much detail obscures the overall visual impression and can make the concept confusing to look at. This is true for any form of visual communication, including illustration.)

Lastly, a concept, once implemented may be required to work well from multiple view points. This is especially not true of illustration, where things only need to look good from a single point of view. Therefore, in illustration one can favorably use techniques that are usually undesirable in concept art: distortions, stretched rendering angle, inconsistent perspective or lighting, specific use of colors and highlights and shadows, and others.

Thus illustrations may not work well when used as concepts, because the implementation can be problematic and require more work from the team to make the environments and characters (etc.) look right.