Apparently corporate structure gave him power over the board. Historically this kind of structure was considered undesirable and illegal in a public company, but beginning in the 2000s there was this idea that "visionary founders" should be able to demand absolute control, ex: Zuckerberg
I think many employees have hoped that it would IPO, but Mullenweg was already "post-economic" financially comfortable, and probably wasn't thrilled at giving up more control, or actually being subject to further scrutiny
Being on the board is a Sisyphean effort based on the weighted voting setup. Some seem to be slower to realize this. At that point, they could quit and walk away with nothing, or as others have suggested, this could have been done deliberately to at least get a payday on the way out.