Maybe because tons and tons of software build with GNU make, not with Ninja, and if you want to be able to build that software, you need GNU make?
Also, Ninja by itself is not really a replacement for GNU make. Rather it's a tool one can build such a replacement on, so the comparison is a bit off to start with...
The Cosmopolitan Libc repo uses GNU Make. It builds 672 executables, 82 static archives, and 17,637 object files and runs all unit tests in under 60 seconds on a $1000 PC. How fast would Ninja do it?
It’s old software which has far better and faster replacements like Ninja.
The whole design of Make itself is outdated and inefficient which is why tools like Ninja are much faster.