Why NTSC stands for the National Television System Committee, the group that defined the analog color television standard used in North America for decades. In amateur radio it refers to analog fast-scan color TV, which fills a roughly 6 MHz channel and so is only permitted on the 70 cm band and higher. "Fast-scan" distinguishes it from SSTV (slow-scan TV), which squeezes still images into a voice-width channel on HF.
Watch out The compression-scheme answer is tempting because modern digital TV does use compression, but NTSC predates that and is purely analog with no compression or encryption.
NTSC = National Television System Committee: old-school analog color TV, needs 6 MHz so 70 cm and up.