We wanted something like Blaze (Google's system); at the time we were using Buck which didn't satisfy us (e.g. only one output per genrule, no directories; and it was more or less impossible to write first-class support for a new language without modifying the core system). Subsequently Bazel got released but we still find that a bit lacking in some areas; e.g. the CLI, the JVM reliance and some of Starlark (e.g. it maintains Python 2 syntax compatibility so can't have type annotations).

We'd also tried Gradle previously but that was pretty awful for anything non-Java which made it a non-starter.

To be clear, I'm one of the original implementors, although I imagine that was obvious already...