Some projects let individual contributors retain the copyright on their contributions. When a project is run like this, for better or worse (usually for better) it becomes impossible to re-license it, as it is usually impossible to track down every single contributor and get them to agree to new terms. This is the condition of the Linux Kernel.

Some projects demand that copyright is assigned to the organization up front, so the organization has the ability to re-license or dual-license the code down the road. This might also be done so the organization has standing to sue people who infringe the copyright. Notably, this is a requirement of the GNU project. This has resulted poorly for the end-users of some projects/organizations in the past though (MySQL for instance, which now belongs to Oracle).