The problem is that more often than not, the autotools don't work. They were the go-to tools for the portability problems of the mid-late 90s and early 2000s.

The problem is that more often than not, the autotools don't work. They were the go-to tools for the portability problems of the mid-late 90s and early 2000s. Unfortunately, since then they have stagnated, and they are now stuck solving portability problems very few of us have. But they don't do much for contemporary portability issues, while CMake most certainly does. Most recent problems I've encountered are that Autotools failed to build properly with MinGW and Cygwin. CMake worked perfectly. Since the decline and obsolescence of proprietary UNIX platforms, the main purpose of the Autotools has been effectively removed. Today, the Autotools work on Linux, BSDs and MacOS, but with pretty much everything else having been long obsoleted, the amount of testing on other UNIX platforms is both minimal and completely pointless. Few people care. They are dead. But, they still don't support the most popular platforms of today: Windows, Android and iOS. But CMake does.