Experimentation was indeed one of the main reasons why it started out as an independent project. Bloop has been the testbed of BSP, compiler performance improvements and other innovations when it comes to client integrations. The cost of experimenting in the zinc repository was too high.
However, the main reason why it was separate is a matter of principle. In my view, a build server is much more than an incremental compiler. Yes, Zinc currently powers bloop, but who knows how we will be doing compilation in the future? Who knows if we find better approaches to incremental compilation than zinc? Or break internals of the test framework APIs?
Bloop only marries itself to its configuration file format, the CLI commands and the BSP protocol and these are independent from the backend. If rsc or a new compiler comes along, integrating it with all the existing build clients (numerous IDEs and build tools) is simpler.
I elaborate on some of these points in What is Bloop · Bloop.