There's a reason for this. It's called time zones.
Flying SEA to ATL is over 2100 miles and about 5 hours of flight time. So a flight leaving at noon Pacific time arrives at about 8pm Eastern. Just in time for the last connection bank. So there's little reason to have other afternoon flights getting in late at night, or after midnight Eastern. Thus, nothing until the red-eyes, which arrive in time for the morning connections at ATL.
You'll find this not just from SEA, but also SFO and LAX -- eastbounds have this afternoon lull for transcons.
There is one DL flight that leaves SEA mid-afternoon, but since it arrives around midnight, there's no onward connection until morning. It's pretty much for local market, and to rotate aircraft back to ATL for maintenance.