Because you can as and because there are big rewards for doing it right and you're in a good position to that.
It's true for any company, and even more true for tech companies. They have to continue to grow for many reasons, including keeping their existing products better.
I personally am not a JIRA fan. We used it for a while and it just really didn't work out for us. The entire experience was too cumbersome and unfriendly. However, that has nothing to do with them growing. If anything, they're doing an amazing job growing with a product that isn't fun to use at all.
I disagree with this completely. Atlassian is in the business of collaboration tools. They've done that for developers, but it doesn't mean they're only capable of making developer tools.
Also, where's the risk here? Other than the inflated price I don't see any risk. Trello isn't just an idea but rather a real product with a massive team and a ton of users who love it. They've iterated on their platform enough so I imagine many of both the technical and experience aspects are polished enough. What am I missing?
Why is that? Why does a company that is good at something NEED to expand beyond their area of expertise? Is is just me who sees this as American Capitalism™? Wouldn't it be better to try to make their products excellent and attract the many developers who are not using their suite? Why the always constant push to grow at any cost?
I'm saying this as someone who has been using Atlassian products for a while. They seem to focus on JIRA and Confluence only, and letting all the other products be 2nd or even 3rd class citizens. Why expand if you cannot keep the software that you already own up to date?