I was an early adopter of CMake (although nowadays I mostly either use plain Makefiles or use languages that have a reasonable build-system integrated) and while at the time it did feel much more approachable than the autotools I remember being baffled by how utterly terrible the syntax was. It's ugly, noisy, redundant, quirky and didn't share any obvious similarity with an existing mainstream language. I wonder how much that cost them in terms of adoption.

At least autotools and m4 have the excuse of all that historical baggage and very broad portability.