Gradle a single-threaded execution in some parts (I believe it may be configuration?), because the build-rules are written in a full blown language with access to the filesystem, and internal parts of Gradle.

In Bazel, rules have to declare inputs and outputs, and this can be enforced with sandboxing. This allows to predict that two rules do not interfere with each other, so we know we can run them in parallel. Our extension language disallows direct access to the file system, and also forbids access to other sources of non-determinism, such as hash tables and the clock.