A guess would be because they market it as a fuel-controller only with everything built in and adaptable to any carbureted engine. Controlling spark timing would be very dependent on the engine family. You can find the Holley Commander 950 manual online to read-up. They only allow controlling spark for a GM 8-pin HEI (1985-95), which requires a special adapter cable. Also much info on the Megasquirt site. Most people who upgrade their A-body ignition use the GM 8-pin HEI since easiest and cleanest install, plus a wicked spark. Even hotter is the LSI coils (1 per cylinder), but you are beyond the Commander 950 then.
