The main reason is that I don't have any inside information on how ximian is doing from a financial point of view. They continue to be one of the most interesting companies in open source from a technical point of view, and Miguel is speaking at OSCON. But it would be embarrassing to cite them as a success if in fact they are just burning venture capital. I know that the companies I mentioned as successes have real revenues; because Ximian is still private and I'm not on the inside, I don't have that same knowledge about them.
They are, however, a very interesting company.