I have to second Footkerchief's answer.
From what I've observed personally:
What it boils down to is control. Right now, things are simple and easy to understand: Closed source, one person, and money goes one place. At this point in the project, which has never had any stage of development opened up, open sourcing or even bringing on additional people opens up a huge can of worms. And what really makes it exceptionally complicated is the implied aspect of money from the donations. If the donation statistics are accurate... there's a ridiculous amount of money moving in the direction of this project. Before money enters into the mix, the project could be open sourced without much issue apart from the original developer getting used to other hands on the project. With money comes problems of all kinds. Namely, who owns the source and the rights to donations?
Open sourcing with a community as ravenous as this (people are mapping out memory regions and data types to use applications with integrated external process memory editors... to add features to the game... that's borderline psychotic in many aspects), it wouldn't be long before the code would practically get overhauled. Then, whatever implied ownership had by the original developer is lost. With that, donations become a touchy subject as to whom people are paying? Even with restrictive licenses, the processes and internal structure of the game would be reversed and re-engineered to an open license codebase. The copyright applicable components (storyline, unique names, and such) would be changed over to open licensed versions. So effectively, the game would leave the fortress and never come back.
With limited outside contracting, there's a HUGE mess of legal paperwork to go through to protect the project. Non-disclosure, copyrights, and other things... It gets REAL messy, REAL quick. NDA's are time-limited and hard to enforce if it prevents a developer from getting work and copyrights only protect from direct duplication and derivatives. Without a savage legal team, things would get out of control quick.
So... if control wants to be maintained and the income attached to it... This is it folks.