Because Jira is flexible and has the needed features and integrates with most things you care about. To the point where everyone in the org can tolerate it. Alternatives tend to focus on one group of users and the rest HATE the product.
I've tried a lot of these products and in the end come back to Jira because it works better on average for everyone.
This comment nails it - anyone who has had to investigate other offerings that satisfy the needs of project management, design, and engineering will quickly find that all the other options out there are total garbage in comparison to Jira, which is why Jira remains at the top of the totem pole for what it does.
As an engineer & former manager, there are definitely things I didn't like about Jira like slowness, but everything else I investigated/used were worse in multiple significant ways.
There's a worse world - one where you have on prem Jira, Monday, Paper, O365, Confluence, Github, and internal api documentation all out of alignment with the other. I'm begging to just end my misery.
The other thing about jira is it’s configurability. I look for replacements every so often and all will require us to change our process. We have adapted jira to our exact process like I’m sure many teams have.
I've tried a lot of these products and in the end come back to Jira because it works better on average for everyone.