I don't think it's about "rejection" but a practical thing.
When starting a project you may need some 3rd party stuff (library, framework, etc) that doesn't support Python 3 (yet; for different reasons, sometimes the project is unmaintained, sometimes is a _huge_ project and Python 3 support is not ready, but I have yet to find a project that "rejects" Python 3).
Because 2.x (2.6 to 2.7) is available, it's not a big deal and you just use Python 2. Thanks to 2.7 (that includes several features from Python 3), finally moving to Python 3 shouldn't be a pain for most people, once all your requirements support Python 3.
When starting a project you may need some 3rd party stuff (library, framework, etc) that doesn't support Python 3 (yet; for different reasons, sometimes the project is unmaintained, sometimes is a _huge_ project and Python 3 support is not ready, but I have yet to find a project that "rejects" Python 3).
Because 2.x (2.6 to 2.7) is available, it's not a big deal and you just use Python 2. Thanks to 2.7 (that includes several features from Python 3), finally moving to Python 3 shouldn't be a pain for most people, once all your requirements support Python 3.