(Disclaimer: worked in this area, haven't in a few years, my facts are sketchy.)

Bazel is open-sourcing of the build system used to build the majority of code within Google. Chrome (in part because it targeted Windows) didn't use the standard build system and instead has had its own series of custom replacements. gn is the latest iteration on that. As other commenters have mentioned, Bazel expects your code to be organized in the way Google code is, while gn likely makes similar assumptions related to how Chrome is organized.