In order to Xmake reach the mainstream, it needs to be supported by major IDEs. The major advantage of CMake over all other building systems is the broad range of IDE support. One can open CMake project without even touching the command line with Visual Studio Code, Visual Studio IDE, Eclipse IDE, Clion or Qt Creator and immediately get support for code exploration, code completion, refactoring and so on.
Meanwhile, Clion is built around cmake. Why would any Clion customer want to drop first-class support for cmake to fall back to an unmaintained plugin that seems to be abandonware?
But they are not officially supported plugins, while CMake has official support from major companies and IDE makers, namely Microsoft, Jetbrains, Qt Company. Further comments in this threads also pointed out that some Plugins for Clion are abandonware and may no longer work.
A reason for using Cmake is that this build system's CMakeLists.txt also works like a universal IDE-agnostic project file that saves the user from wasting time tinkering IDEs and lots of XML files in the case of Visual Studio MSBUILD building system.