Because Google didn't open source Blaze soon enough, and Xooglers who moved to other companies wanted something similar and ended up building clones (Buck and P

Because Google didn't open source Blaze soon enough, and Xooglers who moved to other companies wanted something similar and ended up building clones (Buck and Pants come to mind). Eventually, Google did release an open source version of Blaze called Bazel, but by then the eco-system was already rife with many other similar but slightly different build systems.