Kernighan (and P. J. Plaugher) had written a book called "Software Tools". It was supposed to give several reasonably-lengthy examples of software that did actual useful functions. It was written in RATFOR, which is a pre-processor for FORTRAN. Some time later, they re-wrote the book to use Pascal, calling it (predictably enough) "Software Tools In Pascal". After writing it, Kernighan wrote this paper, basically because he was thinking "That should have been way easier than writing the same stuff in RATFOR. Why was that so hard?"
I used Pascal for two years professionally, and many of the issues in the paper I ran into. Pascal was just clumsy to use. It was a good teaching language, but not good for professional programmers in many cases. (C, on the other hand, was written by people trying to write an operating system, and turned out to be a decent language for writing operating systems in.)
Note well: All of this is true of the original Pascal. Things like Turbo Pascal improved it and made it actually a usable language. But even that wasn't portable - there wasn't a Turbo Pascal for anything other than the IBM PC, so far as I recall. And every other "improved" version was different from Turbo Pascal, so there was no portability between extensions either.