Buck2 is IMO much better than even Bazel is from a design POV, because it actually cleanly separates all user rules from the build engine, and has a coherent modern design around a sound theoretical basis. Neil, one of the leads and author of this post, has written many build systems, so it's not like he's unaware of Bazel; his taxonomy of build systems and ones like Bazel in "Build Systems a la Carte" is worth reading even for Bazel users. It also has a snappy UX and is fast on the command line, which Bazel still lags at a bit. Bazel has all the mindshare, though, and a lot of good features and libraries. But Buck2 is in a different league from Buck1 completely, and in many ways even from Bazel, IMO.

Even then, it's not like rewriting a billion lines of BUCK files to use Bazel was even practical. Realistically any solution had to have a direct migration path from Buck1 without rewriting everything. I don't even work there, this is just pretty obvious from the design constraints and talking with the dev team, though. Frankly, I'm pretty impressed they were both able to meet the goals they had (migration from Buck1, better performance, more extensible), while still jam-packing the thing full of good design decisions and features like they have. It's good work.

(I am a pretty happy, non-Facebook user of Buck2 already, FWIW.)