Because it's a graph database so it doesn't really fit naturally with SQL.
Competitors like OrientDB use something that's nearly SQL but not quite, and it's the discrepancies between the two that catch you out. Arango's query language is pretty nice when you get used to it.
The one criticism I have of Arango is that they're doing a lot of wheel reinvention with foxx, when I think they'd be a lot better off leveraging more of the node ecosystem.