I’m a little biased, but as someone who doesn’t want to spend much time dealing with systems administration types of tasks, there are lots of benefits to going with a hosted Discourse site. For example, it means that you don’t need to consider the issue that is asked about here: How to use Cloudflare to protect a Discourse site. Hosting will also deal with the configuration of incoming and outgoing email for you.
I’m also unsure about the assertion that most Discourse instances are self-hosted. At least in terms of activity, I think that many of the busiest Discourse sites are hosted.
That said, self-hosting has some advantages. For a small site that is just getting off the ground, it may be the perfect solution. Other than being affordable, it also gives you the freedom to install any plugin that you wish. That means that it also gives you the freedom to break your site. Depending on your use case, that may be an acceptable risk.
Even given what I’ve written above, I don’t think I’d be interested in Discourse if it wasn’t open source software that anyone could host on their own server. For self-hosting, you can get lots of good support from this community.