Please help me understand this better. So HStoreField is not exactly Json field type of postgres but rather a subset of it, i.e just string->string mapping without any nesting. You cant even have key->array mapping?
However hstore has been a part of Postgres since version 8.2 released in 2006, and I think was around as a third-party project before that, so it's used by a lot of projects.
It's also easier to manipulate hstore values then json values within the database. hstore's simplicity allows operators for combining and subtracting hstore values[0] which json doesn't have. Due to its restrictions, an hstore can also be converted to and from a record type (think database row). JSON fields on the other hand will generally be populated by your app and then only queried by the database.
[0] http://www.postgresql.org/docs/9.4/static/hstore.html#HSTORE...
However, the author of contrib.postgres opened this PR yesterday, so you can certainly expect them in the next release: https://github.com/django/django/pull/4427