It didn’t support in-tree configuration files, so modifying the CI structure was hard, as we can’t test it along the commit. The other thing is that we didn’t have enough people to maintain the infrastructure and it was unstable. TaskCluster has its own maintenance team, so it’s better than one or two Rust team members managing the entire buildbot infrastructure.
Along with that, as a part migration to SaaS it brought some side products, like nightly from artifacts instead of cron build. Those are described at Rust CI / release infrastructure changes.