In order for such a big project as Yii to function properly, we need core contributors watching it constantly: making sure it goes in the right way, managing releases, making decisions. Time proved that while it is possible to do it having a full-time job, it is not as focused, fast and committing as needed.
Previously it was possible to support Yii in many ways: contributing code, participating in community activities, translating documentation. Donating to the project was not among them. Now it is.