This is a supercomputer meant for academic and research use primarily. The computer thus must support a wide array of users, who'll be working on an even wider array of software, much of it very much not optimized for anything in particular. Now, the FZJ that hosts this machine has a department that aims to help researchers optimize (and professionalize, i.e. tests and CI and such) their software. We can help optimize your tool for our particular machine, or some other hardware, and we make some tools that help you use supercomputers (some of our users, perhaps many, are not trained software developers at all, let alone habitual HPC customers).

So, the hardware needs to be general purpose, because the software it's going to run are not known yet, and much of it will be barely executing ("research software" ;)).