It's only "half the price" if you ignore the cost of Confluence itself. The Atlassian people are also historically awful at building out features that make hosting their products easier (for example, they've ignored a request for a "read only" mode for about 12 years now [1]).
The simple fact that it is a product that is made by someone other than Atlassian is a huge selling point.
[1] https://jira.atlassian.com/browse/CONFSERVER-6390?page=com.a...