Mostly the fact that I already had an instance up and running for chatting with some people and because it was pretty trivial to integrate Uptime Kuma with it.

Mostly the fact that I already had an instance up and running for chatting with some people and because it was pretty trivial to integrate Uptime Kuma with it. Sending e-mails would have also been a decent idea, but honestly configuring a Mattermost app/account/token is easier than messing around with a new mail account and all of the SMTP settings. As for Mattermost itself, previously I also enjoyed running Rocket.Chat, though somehow Mattermost felt a bit more like Slack (what I'm used to), runs with PostgreSQL as a backing store and also has the Boards and Playbooks integrations, alongside lots of other goodies. I do miss the ability to integrate Jitsi or something else in the actual interface like Rocket.Chat did, but it's not really a large problem overall.