The why is simple: *only* folk that are specified as uploaders by Ubuntu are permitted to upload code to Ubuntu; granting write access to branches that the distro intends to be able to build from would permit a backdoor.
The why is simple: *only* folk that are specified as uploaders by Ubuntu are permitted to upload code to Ubuntu; granting write access to branches that the distro intends to be able to build from would permit a backdoor.