Understandably, there are things that people really want to see in SMBX. If the game were to be open-sourced those people would be able to implement the idea themselves. They would need to know how the coding works, and be able to supply the graphics.
Once people have finished adding their elements, they can send it to a developer, describing their changes. If the changes are good enough, the developers can update the download link with these new changes.
If SMBX is open-sourced, I'm sure it may be updated more frequently than usual.