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

Automattic also isn't a public company.

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