Because most free wiki software is kind of bland and terrible. Don't get me wrong, they are amazing for what they are but they don't scream "professional". But

Because most free wiki software is kind of bland and terrible. Don't get me wrong, they are amazing for what they are but they don't scream "professional". But actually that's not the key point. Nobody buys just Confluence. That would be silly. A bland and terrible (but free) wiki software is definitely better than Confluence. People buy JIRA. And then you've bought into the Atlassian ecosystem, and you want the nice tight integration with your wiki software