They say right there that Fortran, PL/I, and Algol 68 were too big and complicated for Unix. Yes, if you are building a system, it is more productive to use a language that is built for purpose and pleasant to work with ("fun") than one you have to struggle against all the time.
> Although we entertained occasional thoughts about implementing one of the major languages of the time like Fortran, PL/I, or Algol 68, such a project seemed hopelessly large for our resources: much simpler and smaller tools were called for. All these languages influenced our work, but it was more fun to do things on our own.