Though I don't know the specific project, I can tell you that it is common for projects to reject contributions under licenses that cannot be subsumed by the project's main license.
For example, if a project under the Apache License 2.0 were to accept a contribution of CC-BY-SA code, the complete product could no longer be available under the Apache License (and other licenses with similar terms) -- causing problems for downstream consumers.
For example, if a project under the Apache License 2.0 were to accept a contribution of CC-BY-SA code, the complete product could no longer be available under the Apache License (and other licenses with similar terms) -- causing problems for downstream consumers.