Because it says:

> Plankalkül was more comprehensively published [vague] in 1972. The first compiler was implemented by Joachim Hohmann in his 1975 dissertation. Other independent implementations followed in 1998 and 2000 at the Free University of Berlin.

So it was never implemented until mid-1970s, which likely made its design process less constrained than languages that came soon after it in 1950s etc.

Because it was more a theoretical exercise.

The later languages were much more constrained by actual implementations.

On the other hand the later implementors realized that some convenience is needed, for example even the earliest languages (like autocode) had variable names instead of variable numbers.

Part of it was that they weren't bound by the limitations of early computers. Those later languages were heavily stripped down because that was all that could fit on a machine made out of vacuum tubes and mercury delay lines.