Because by being open source, this project would attract contributions from different people with different visions. MC would get much more attention. Many contributions would just come to you and you would only have to review them, which would be easier for you.
Now, if someone has a good idea and you do not like it, you won't work on it. But if you also would get the implementation of that idea, it would be easier to include it in the project. MC would improve a lot, and faster.
It can be remotely be compared to the case of Double Commander. It was open-source before, but at some point it took a big step and moved the development to github and since then, the project benefited from more attention from contributors. A lot of problems were and are fixed.