Google owns WebM and unilaterally makes the decisions about the direction of the format. H.264 is a standard and driven by a standards committee and process comprising numerous companies.
The code may be open source but the format/spec is 100% controlled by Google. Forking the code won't help you change the direction of format development. If anyone besides Google wants to make changes, they'd have to fork the entire spec under a different standards body.
As you would with any standard. If h.264 were open source you couldn't fork it and have it work with existing h.264 devices. It would be a new standard.
