Postgres not smart enough to combine fields in WHERE and ORDER BY for index scan?
It has grown smarter in v10.  

I think the reason it wasn't smart enough before is described here:  

https://www.postgresql.org/message-id/1788.1481605684@sss.pgh.pa.us

I don't know how to summarize that, so I'll just say "booleans in indexes are confusing"