Because the flights list the fare buckets available for the total itinerary, even though it shows results leg by leg.

The second combo of flights includes an RJ. Even if there were seats in front on the ATL-TUS leg, "F" and "A" buckets would still show up as empty for BOTH flights. Similarly, the fare buckets for coach are dictated by the leg that has the least availability.

If you want to see what's available (seats up front for a possible upgrade maybe) you need to enter each leg individually.