The main reason is exactly as the article states, maintainability.
> As time wore on, our technical debt finally began to come due. Compilers like Wasabi and their associated runtime libraries are highly complex pieces of software. We hadn’t open-sourced it, so this meant any investment had to be done by us at the expense of our main revenue-generating products. While we were busy working on exciting new things, Wasabi stagnated. It was a huge dependency that required a full-time developer — not cheap for a company of our size. It occasionally barfed on a piece of code that was completely reasonable to humans. It was slow to compile. Visual Studio wasn’t able to easily edit or attach a debugger to FogBugz. Just documenting it was a chore