The choice to design a web development framework depends on the interest and skills of the people who wants to build frameworks. For the most history of web application development people have used "scripting languages" and thats why most of the frameworks are based on those languages, also these languages are "productive" and good enough for implementing the typical CRUD layer over a database which is what most web apps are about.
Check out
https://github.com/ipkn/crow for a Modern C++ web framework.
NOTE: I used to do C++ 5-6 years ago and now getting back to it by going through modern C++ and also the idea that learning C++ as a layer on top of C is a bad bad idea to learn C++.