There's also a technical reason, which is that the build system is written in the language it targets. So the cool tool is written in coolang. That's obviously not required, you could use any programming language for the cool tool, it just happens that all people that care about the cool tool, understand the needs of the ecosystem, have issues with missing features etc. already have a non zero intersection of languages they know of: they all know coolang.
If coolang decided to try to add coolang support to Bazel instead, they would probably have to learn Java[1]. Current maintainers or contributors to Bazel don't know coolang, and they don't care about it much, especially in the early stage. And maybe coolang developers don't know Java, or even actively hate it with a passion (that's why they were on the market for a new language). And even if some coolang developer decided to contribute to Bazel, the barrier would be much higher: being a mature build system with so many features and different needs, surely working in it is going to be complex; there will be many different concepts, and layers, and compromises, and APIs to work with. So for them it just makes more sense to use coolang so that all coolang developers can contribute to it having a real need for the cool tool to improve.
[1] I know nothing of Bazel. So just bare with the example even if it's technically not correct.
A nit (hopefully a welcome one given that it supports your statement) is that Bazel's rules are written in a language called Starlark, which has python syntax just without classes and a bunch of limitations surrounding switch statements and loops.
The core of your point is correct: who wants to both support an additional tool chain and an additional language for building things? Terrible sell.
Go itself is a little bit of an edge case because they recommend leaning on Make, but ironically they do not use Make for its intended purpose and all the (actually good) functionality that Make gives you is reimplemented from within the go compiler.
If coolang decided to try to add coolang support to Bazel instead, they would probably have to learn Java[1]. Current maintainers or contributors to Bazel don't know coolang, and they don't care about it much, especially in the early stage. And maybe coolang developers don't know Java, or even actively hate it with a passion (that's why they were on the market for a new language). And even if some coolang developer decided to contribute to Bazel, the barrier would be much higher: being a mature build system with so many features and different needs, surely working in it is going to be complex; there will be many different concepts, and layers, and compromises, and APIs to work with. So for them it just makes more sense to use coolang so that all coolang developers can contribute to it having a real need for the cool tool to improve.
[1] I know nothing of Bazel. So just bare with the example even if it's technically not correct.