Have a look around and read the history of the Mattermost project. You’ll find it’s developed primarily by a private company that made it first for their own use. They are an established firm with a workflow and tools for their team. I’m sure Jira is something they use in their normal closed source software development cycle and using the tools their team was already on mad the most sense. Opening up the issue tracker and migrating parts of the workflow into the public space is something they’ve obviously been working an as this tool has gotten open source traction, but ever so there is no particular reason why they should switch issue trackers when their team that is working on this is already tooled up with something else.