There are a number of factors that could be affecting the quality of the video stream. The streams from Youtube and Hulu are likely better at using compression/buffering to overcome connection issues that could be affecting your home wifi (assuming your Chromecast is using wifi), your internet service, or the bandwidth available from the content server (whoever the pac-12 pays to store/serve their content).
It could also be a Chromecast optimization issue. You could try testing the quality of the streaming service on other devices on the same network to see if they perform better. If you get better performance on a different device, the issues are probably with your Chromecast. Otherwise, it is probably an issue with the service from the PAC-12.