“Reimplementation from scratch” seems like a generous description of Buck. It was initially so like Blaze that I always assumed a xoogler exfiltrated at least the documentation of Blaze.
I really can't speak to that in any way, I was just trying to answer the OP's question as to why there are a number of--on face very similar--Bazel-style build systems. It certainly seems like a fair assumption, though.