Is there a reason why the source code of softwares mentioned in research papers is not released ? I understand that research papers are more about the general idea of accomplishing something than implementation details, but I don't get why they don't release the code.

For example, in this paper ends with:

Results

The human line drawing system is implemented through the Qt framework in C++ using OpenGL, and runs on a 2.00 GHz Intel dual core processor workstation without any additional hardware assistance. We can interactively draw lines while the system synthesizes the new path and texture.

Do they keep the source code closed intentionally because of a monetization they intend to make with it, or because of copyright ?