There is absolutely no good reason for them to open source it. It's more profitable for them not to. And lets face it, business is about making a profit. Not to mention, there's less headaches when certain people want to create a branch, so you have a million different forks and incompatible versions, or the community takes so long to decide on a uniform change that progress moves at a snail's pace, and some other standard overtakes it, ala OpenGL.
Nope, open source is entirely too overrated in many situations. Closed but free to use is perfectly fine if it gets the job done.