Because Github is, for the time being, what everybody else is using. You can't go wrong choosing Github in terms of users and contributors being familiar with the interface.
That said, I only use Github as a mirror. My "master" repositories are on a private, co-located server. The beautiful thing about Git is that it doesn't require any special software on the server side for hosting--just the regular client. And in particular, Git doesn't require a special daemon service. It doesn't even require a special service for hosting read-only HTTP repositories. The stock Git client can clone repositories over HTTP from static site files. My "official" public Git URLs are just HTTP URLs to a static read-only mirror which is updated from a Git hook when pushing. Those HTTP mirrors required no additional configuration whatsoever; they just sit inside my public_html/ user tree.
The secret to running your own server over the long term (I've been co-locating a server for almost 2 decades) is to keep things simple. It helps that I use OpenBSD, which has always maintained default web and mail servers which, even if disabled by default, are still "secure by default" in the sense of the stock software and configuration being well thought through. (This is especially true since OpenBSD replaced sendmail and Apache with their own software.)
I originally used CVS before switching to Subversion for a few years. But I was never comfortable exposing even a read-only mirror until I switched to Git, as both CVS and Subversion required special server-side software that I was unwilling to run, let alone maintain.
I only started using Github because most other people prefer using Github's interfaces for forking repositories and submitting pull requests. It's a trivial cost on my end (just an additional push), so I don't mind.
GitHub's UI is far superior, for one. You're also placing your code in an ecosystem of other OSS projects, which I believe is a good thing.
Also, GitHub private repositories for an individual are far cheaper than running a H/A GitLab on DO, AWS, Linode, or any provider worth mentioning. By definition of running a H/A solution on DO's cheapest offering, you're paying $15/month for three VMs, and we haven't started talking about backups yet.
I'm against self hosting when it makes no sense to do so, and only because I've seen people get burnt by the process and I don't want to the same to happen to you. I'm probably capable, but many aren't.
Legal or regulatory reasons are the only reasons I can think of that might force your hand in to self-hosting.
