Because Shimano did not make them.
In the late 1980's Shimano began bringing out compelling groupsets that included compelling innovations such as indexed gears, BioPace chainrings and improved brakes. The brakes had a spring in the lever and a lighter spring in the brake making them much easier to use. They also pioneered hidden cables from the brake lever.
At the time the Yen was low and Shimano products competed very well on price.
Furthermore, Shimano did a 'Microsoft' on the OEM market, to use the indexed gears you had to have the whole groupset. There was no mix and match like you have today.
As a consequence of Shimano's improved technology, the value of the Yen and their sales tactics of complete groupset or nothing, their competitors had a tough time. This led to consolidation in the European component business leading to the SRAM company that we have today.
The dual-pull levers pictured were most likely to be a Dia-Compe design made by Weinmann. Weinmann brakes were de-rigeur in Europe OEMs until the Shimano era of market domination. They lost their footing in the marketplace and haven't been seen since the early nineties.
These levers were marketed as 'Safety Levers' and they were a requirement for bikes imported into the USA for a time. Exactly what the law was requiring the 'Safety Lever' and when it was no longer a requirement is a matter for further research - not a lot exists online.
In time the dual-pull levers emerged in copies from the far East. The Dia-Compe/Weinmann patent on the dual-pull lever may have expired making the way for clones. These clones appeared on bikes after the demise of Weinmann and are of inferior quality to the originals with chrome parts rusting up pretty quickly.
Here is a picture from the 1983 catalogue: