Why C and C++ standardization flourished up today and Pascal stopped in 1991?
That's a good question and I doubt anyone can offer a definitive answer. My guess and, that's all it is, a guess, is that the Pascal language has never been targeted in production to a specific area of computing.
If we compare it to C (some people won't like the comparison but... oh well), from its conception the language was meant to be a systems programming language, a "high level assembler" as it has sometimes been dubbed. While the design of the C language is a _genuine atrocity_ (unparalleled until C++), even those who strongly dislike it (myself among them) must acknowledge that once you learn how to use it, it can be a very effective tool for someone doing systems programming.
If we compare it to COBOL (yes, there are still plenty of COBOL programs out there and new ones are _still_ being written), that language was designed to be mostly self documenting and meet simple procedural business requirements. In spite of a very large number of deficiencies for other uses (and some even for its primary use), it fulfills its goal really well, much much better than C or Pascal or both put together in that area.
And for those who enjoy and extoll portability, the _first_ portable program was written in COBOL and, well written COBOL programs are very easy to port from one architecture to another.
The story is the same when we consider FORTRAN. It lacks many capabilities but, for pure raw number crunching, it packs a punch no other "somewhat general" language has. For that reason, there is a 2018 standard for the n-th iteration of the very first compiler.
Pascal was the first and, possibly last, not overly focused language designed to teach, not programming but, _how to program_, i.e, use and apply the structured programming methodology with a language that elegantly supports its constructs. It fulfills that purpose quite well. The problem is, that methodology can be taught using other languages (particularly when today's "structured programming" includes dynamic gotos), as a result, Pascal's claim to fame is no longer unique and, it has no other one (at least not so far).
Because their purpose (C, COBOL, FORTRAN, others) is clearly defined, there is continued interest in improving them to make them better tools to accomplish their purpose. Pascal's purpose - to teach structured programming - doesn't really require updating the language. The answer is: learn using another language.
Until someone clearly states what Pascal is for (other than to learn programming) and, starts extending its _design_ instead of just bolting junk found in other languages to it, it is quite unlikely the situation will change. To add to the problem, somewhere along the line, Borland figured they could make some quick money by adding junk into the language instead of making the language better. There is still no visible light in that tunnel which is not a good thing for Pascal.