Another problem is deployment, if I adopt some RDBMS system to my needs, then I already have a bunch of web hostings supporting this type of DB or I can use installer to deploy the DB, run a few scripts and everything is ready to run.
But you and I appear to have very different needs.
GraphDatabaseService graphDb = new EmbeddedGraphDatabase( "var/graphdb" );
Of course, Neo4J is an embedded solution. It's quite different than using a separate server application.