Most open source project take longer to develop. Taking Gimp for example, it took 6 years to develop the last version. Usually, open source projects that manage to keep a steady pace in development are usually supported by a core team of developers tied to a company.
Making OB open source wouldn’t do any good. The first problem that comes to mind is that anyone who wants to start contributing, has to get up to speed on the whole codebase, and that takes time, you can’t actually hack away your way through coding new features. More contributors also mean it would be harder to enforce a particular coding style and code reviewing would take longer.
If you take into account that there’s a possibility that they use a custom build system, they’d have to release that also, and make sure that it’s compatible with different machines which would involve additional development not related directly to OB.
There are a lot more reasons why making it open source wouldn’t help at all and actually make things worse. Someone in this thread pointed to “The mythical man-month” before, and if you’re not familiar with this concept, you should check it out, it’s a really interesting read.
Also, while open source as its perks, it’s in no way a viable solution for everything and everyone, and it should never be a solution to development related problems.