The reasons are very simple to explain.
In 1972 a well known tech-company brought the world's first scientific pocket calculator. Other then most business calculation machines (with the "+" on the right bottom near the "=") this calculater uses simply postfix notation (aka RPN/Reverse Polish notation) with a keyboard layout that was optimized for engineers and scientists (more Information about this in the substantiation for the IEEE Milestone).
This design was typical for HP-Calculators for many years (the last calculator with this layout was the HP-41CX introduced in 1983 and discontinued in 1990). The keyboard-layout of the DM41X was designed like this calculators.
Later HP changed the design and oriented themselves towards the competition. After this they brougt a RPN-Calculator with a Design which more looks like a "normal" AOS Calculator (i.a. the HP42s). The DM42 got his layout from this calculator.
Some Companys (like Commodore in the 1970th or Casio/HP10s or Numworks) uses a third Layout */ and +- in different rows.
All this layouts have people who like them, people who hate them and users who tolerate them because they want the functionality.
I was afraid that the DM43 came with an other different design which was a new design that mixes the scientific-calculator Key-Position with the business-calculation Key-Order and combines their disadvantages and can so be hated by fans of the old and the new design. Some cheap chinese calculators are designed that way - loveless, the main thing is that it works somehow.