Some projects done by companies, or other individuals, require copyright assignment on the part of their contributors, so the copyrights will remain under the same copyright owners, who may then be able to relicense their code under different terms. Many people have been reluctant to contribute to projects that require such copyright assignment, because they would like to keep their own copyrights. As a result, this undermines the Bazaar model of development and the atmosphere of community in general.
Copyright assignment is a complete non-issue with projects licensed under permissive licences (a.k.a BSD-styled ones), whose licences allow the copyright owners, or every other party, to sublicense such a project.