[supercat]

The answer lies in the 60 Hz AC line. The 30 frames per second / 60 fields per second was selected to avoid having a heterodyne beat causing a moving dark band across the screen. The chroma subcarrier value was constrained to work with the frame rate/line rate.

Black and white video was broadcast at 60.00fps to prevent beating against the AC line frequency (hooking old television sets up to some computers or video games works poorly because many computers have a frame rate that's off by about 0.1Hz). So why choose a chroma rate that causes the field/frame rate not to be 60.00/30.00Hz?

[Myrddin]

See the link to analog television technical details. It appears the consideration for the 3.58 MHz value has to do with the bandwidth of the quadrature modulation of the color information. The modulated frequencies that appear above the subcarrier frequency will bump into the sound subcarrier at 4.5 MHz. Look at the detailed consideration that was made for human visual acuity vs the encoding of the signals for NTSC.

[Myrddin]

Look how the video is sandwiched between the video carrier and audio carrier.