I believe it's because a 60Hz refresh rate is needed to maintain the 30fps video frame rate for tv's. A 120Hz tv multiplies the 60Hz by two (displays each frame twice) to smooth out motion. Cinema is shot at 24fps but needs to be "sped up" to 30fps for display on tv's. A blu-ray movie can be displayed at 24fps on tv's that are equipped to handle the signal (most, if not all, modern tv's can now). This is, of course, a very simplistic explanation.