Another factor is I believe the IATA maximum connect time on domestic itineraries is 4 hours and on international itineraries is 24 hours. I can't recall ever seeing aa.com price out a wholly domestic itinerary with an overnight resulting in a connection in excess of 4 hours. (I have seen these show up during IRROPS, at which point any viable and reasonable routing is generally allowed.)

It likely offered the BOS overnight previously as that's international to/from Canada. I suspect the inventory you need is no longer available or the flight no longer exists, but we'd need more specific to check on that.