Thing is, once you figure out how to make JIRA and Confluence work for you and your organisation, you probably won't want to use anything else ever again. They're really powerful, really good products, with a pretty steep learning curve.
Atlassian's success is because existing, confident, users of JIRA and Confluence do two things:
1- They tell their friends, colleagues, and random people in the street to use JIRA and Confluence.
2- When these users change jobs, they bring JIRA and Confluence with them.
Put simply: it's word of mouth that has allowed Atlassian to grow without a traditional sales force.