Secure browsers want interoperability and for there to be zero objections on those grounds. Companies want to offer a standard web browser, but they need to harden deployments for specific threats. You can see articles like https://aka.ms/EdgeSecurityWhitepaper/Docs for Edge which describe the extra layers of security you can apply while still using the same browser.
Atlassian would want integration with their backend products to increase lock-in and provide a place where their products are centered. IT control how products are presented to end users in organizations that matter (in terms of sales volume.) Establishing visibility and driving engagement is hard if the Atlassian tools are a niche and they want to attack SharePoint or other products. Being able to more efficiently use the tools the company has bought is attractive (even if not a reality.)
Making their browser incompatible is a bad outcome for them because it's an IT choice to adopt their browser. This carries visibility and risk for IT who could be embarrassed. Any backlash carries over to other Atlassian products or affects renewals.
> Companies want to offer a standard web browser,
I don't believe that in a long term. If atlassian creates an enterprise-managed browser they can charge for, there will be a big incentive to making their suite work better in that browser only. Or JIRA/Confluence features will be released using APIs only available there. It will be their EEE.
If they really cared about actual security, they'd optimise their services enough to use them with JIT disabled. And maybe push the industry to do the same. And publish some SSO auth standard that integrates with the browser.
> Any backlash carries over to other Atlassian products
Atlassian doesn't care about users and what they think. If they did, markdown textboxes would still be there and JIRA wouldn't be a slow abomination. But they sell to businesses, not users. So instead of fixed issues or QoL improvements, I get an AI button.
> there will be a big incentive to making their suite work better in that browser only. Or JIRA/Confluence features will be released using APIs only available there. It will be their EEE.
That just sounds like going back to making thick clients/desktop apps vs. web with extra steps. They might as well make their own native Jira app instead of making an entire web browser and breaking their web app to only work in their new browser.