There's other stuff out there. You don't have to use it except when you're working with someone who does and there are other systems with some compatibility for that exact scenario.

Git became popular because Linus Torvalds created it and it's used for the Linux kernel. If it can succeed there then it's battle proven for sure and that's why people pick it. Almost no one is working at that scale though so don't sweat it. Use what you want.

Mercurial was pretty nice back in the day and I think people still use it. If I was going to use something other than git these days, I'd probably take a look at fossil. That thing is so damn cool. You don't even need something like github with that because it's literally built into the fossil binary. And when I say binary, I mean a single, standalone binary for the whole thing.