But can you actually name a competing product that does a better job?
I'll admit I've not looked particularly hard, but as far as I can tell nobody has a competing product, so Atlassian basically owns the whole field at this time.
There are lots of good modern alternatives to Jira. The reason people still use them is the same as why Oracle DBs are still so popular – they have a large sales team and corporate contacts, and so their products are pushed down your throat.
My whiteboard and the sticky notes it holds outperform JIRA all the time - 100% uptime, clear and obvious process flows, haven't needed a service ticket ever.
I'll admit I've not looked particularly hard, but as far as I can tell nobody has a competing product, so Atlassian basically owns the whole field at this time.