Here are some reasons for not picking TaskCluster for our CI:
- TaskCluster is not very accessible to non-Mozillians, and one of our main goals is to have open infrastructure that others can contribute to. It is not practical to run one's own TaskCluster instance.
- Travis and AppVeyor are well-known to open source contributors, so they can more easily contribute.
- TaskCluster is a complex system that is managed by another team who are mostly beholden to Firefox, while Travis and AppVeyor are relied on by diverse customers
- I've not enjoyed writing TaskCluster scripts in JavaScript in the past, though maybe it has other bindings that I would feel more comfortable with
There may be a role for TaskCluster in the release process yet, motivated by Firefox releng's work on creating secure workflows in TaskCluster.