I note that the Suntour shifter is an Accushift and thus still post indexed shiftings introduction. Even the bar mount SA trigger on the Norman is an indexed shifter so the common pre indexed bar mount derailleur shifters seem to have been bar end and MTB thumb shifters.

A combination of industry conservatism, NIH attitude for outside designs, cable housing compression and the difficulty of doing a design which actually allowed shifting from the drops and hoods riding positions as long as the rider needed to both push and pull a lever, or operate a lever pair, accurately for friction shifting made down tube shifters the simplest choice for manufacturers. Indexing made the riders job easier and required less shifting finesse than friction shifting. This allowed th development of combined brake and shifter mechanisms.

Dual levers as on the Suntour command shifters might have helped but I wonder how they actually compared in convenience with Shimano and Campy brifter designs once they were available. I also wonder about the convenience of trying to use them in friction mode. Any riders out there with experience with them who want to comment.