Because TVs didn't have high quality video inputs. Old Apples, C64s, etc, came with a "tiny tv broadcaster" (RF modulator) that broadcast the signal on channel 3. In shit NTSC or PAL quality, which is lower resolution than what the CRT can display.
I understood that. I'm old enough to remember VCR's / game consoles required you to turn to channel 3 or 4. But what were the older, non-personal terminals using? Was there some sort of RF modulated raster pattern optimized for displaying text? Were they drawing everything as vectors? Did they use off the shelf CRT's?
