The C language was not suddenly specified first and then made. C language was developed based on B language, which was a simplification on BCPL systems programming language, and C evolved a few years with new features that were needed and it became popular.
These languages (BCPL, B, C) were used to make the system level tools and utilities and thus were compiled on very small systems and thus needed very simple and they were only needed to support strict lexical parsing to be able to do what compilers do - parse structured text and output binary code. They were intended to have some medium level language so it was not necessary to write everything in system specific assembly language from scratch.
This is in contrast to other languages like Algol-68, which was an application programming language to produce higher level programs.
When C became popular, people also started to use it for applications, so at that point C programs became more and more complex and only at this point most likely people who were application programmers started to miss the feature.
Over the times computers got faster, got more memory, got improved language parser libraries, etc, so basically nothing prevented this but compiler implementations and code were typically adhering to the de facto standard, which later on became the C89 standard.
Many compilers extended beyond the standard and supported this feature as a non-standard extension before the feature was officially included in the C99 standard.
It was included in C99 because it was already found as a useful feature in Algol 68 and C++ programming languages.
The problem with standards is that even if compilers with extensions existed before, you had to conform to standards to be able to have portable code. If you used extensions, the scope could be limited as different compilers might have divergent implementations how to do things.
For example there might be contractual rules or regulations that mandate how you should write compliant code say for example medical devices or military equipment, and they might not allow custom extensions or might be limited to one compiler that has been validated for the work.
If you were required to write safety critical code using MISRA guidelines, they only included support for C99 in 2012, as the previous standard in 2004 stated that there were no commercial embedded compilers supporting C99 available.
So even if technically possible and available, you may not be able to use extensions outside standards for some reason even if you wanted to, or you may not even want to, if you want to support older systems with older compilers that have no support for C99 or similar extensions.