The US oft has this problem, we tend to be early adopters of technology on a wide scale, so by the time a thing comes along that solves most of the inherent problems in the v1, we already have a wide scale implementation of the thing. This happened with TV color, phones (24 Channel T1 vs 32 channel E1 and aLaw/uLaw), credit cards (mag stripes), and all sorts of other things.

SECAM had some real advantages, but made working with composite signals hard, because of their FM nature. PAL and NTSC are reasonably close conceptually, and frankly so is PAL, you can easily encode PAL into SECAM, because it's mostly the composite signals that are different.

NTSC was originally 525 lines/60 fields per second (odd/even lines) giving an effective refresh of 30 fps, the 525 lines itself was dictated by our 6MHz channel width, and the field refresh by our 60Hz power. When they added color, they dropped the field refresh down to 59.97 to deal with a beat frequency issue between the color subcarrier and the audio subcarrier.