almost all mass produced computers of that era (and essentially even present ones) have much of their timing derived from NTSC, not because they are explicitly designed to be connected to television, but because of component costs. Most of the commonly used VGA video modes are essentially NTSC and the 33MHz PCI clock (used as essentially "global" clock now) of modern PC is still generated by PLL from crystal that usually has some NTSC-friendly frequency. Reason is simple: you need PLL anyway to generate lot of different clock signals, so you can as well use cheapest xtal you can found, and that usually happens to be something that is used in lots of mass-market NTSC equipment. Today there are also some other common mass produced xtals (for USB, Ethernet and such things), but in 70's and 80's only mass-produced things that needed exact timing were clocks and television equipment.