First, I tried Googling the answer and found the following definitive blog post: https://engineering.fb.com/2014/01/07/core-infra/scaling-mercurial-at-facebook/ The ten-year-old write-up, along with some later YouTube tech talks, gave me a starting answer: “because performance.”

But I wanted to go deeper and hear from the original deciding engineers. With the help of a teammate, I posted a question on the ex-Facebookers group asking about the history. I also cold-emailed two of the original engineers working on the project to migrate onto Mercurial - they were kind enough to call me off the record and give personal accounts of the project.