The language of R is S, which originated at Bell Labs in 01976. Python began development in 01989, although Guido didn't release it until 01991. And the top 20 on https://www.tiobe.com/tiobe-index/ are Python, C (01972?), C++ (01982?), Java, C# (01999? though arguably it's just a dialect of Java), JS, Visual Basic (first released 01991, within your window), Golang (02007), Delphi (under this name in 01995 but a dialect of Object Pascal from 01986, in turn a dialect of Pascal, from 01970), SQL (01973), Fortran (01957), Perl (01987), R, PHP (01995, within your window!), assembly (01947), Rust (02006), MATLAB/Octave (01984), Scratch (! 02003), Ada (01978?), and Kotlin (02011).
By decade, that's one language from the 40s, one language from the 50s, no languages from the 60s, 5 languages from the 70s, 5 languages from the 80s, 4 languages from the 90s, 3 languages from 0200x, one language from the 02010s, and no languages from the 02020s.
Lua is #33 on TIOBE's list, but given its prevalence in Roblox (as Luau), WoW, and other games, I suspect it should be much higher.
For some reason, CUDA (a dialect of C++) and shader languages like GLSL don't show up in the list at all.
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I think most of what's going on here is that it takes a new language a long time to get good, and it takes a new good language a long time to get popular. Perl, Python, Java, PHP, and JS became popular because of the Web; https://philip.greenspun.com/panda/server-programming explains why Perl, Python, and PHP did, and of course Java and JS became popular because they were the only languages you could make interactive web pages in:
> You would think that picking a Web site development language would be trivial. Obviously the best languages are safe and incorporate powerful object systems. So let's do everything in Common Lisp or Java. Common Lisp can run interpeted as well as compiled, which makes it a more efficient language for developers. So Common Lisp should be the obvious winner of the Web server language wars. Yet nobody uses Common Lisp for server-side scripting. Is that because Java-the-hype-king has crushed it? No. In fact, to a first approximation, nobody uses Java for server-side scripting. Almost everyone is using simple interpreted languages such as Visual Basic, PHP, Perl, or Tcl.
> How could a lame string-oriented scripting language possibly compete in power with systems programming languages? Well, guess what? The only data type that you can write to a Web browser is a string. And all the information from the relational database management system on which you are relying comes back to to the Web server program as strings. So maybe it doesn't matter whether your scripting language has an enfeebled type system.