Another two reasons are onward connections and efficient aircraft utilization.

An eastbound flight with a 7am departure from the US East Coast would get to Europe at about 8pm local time (7 hours flight plus 6 hours time difference). Few connections would have come in that early in the morning to meet the 7am departure, and few connections would be available same day onward to other European destinations, because not that many EU flight depart late at night due to curfews at their destinations. So for a daytime flight to be viable there would have to be enough of a O/D traffic to fill that aircraft, consistently. NYC-LON works, but few other pairs would. WAS-MAD certainly would not.

And then there is aircraft utilization. If that aircraft that arrived at 8pm in Europe were to turn right around, it might depart for the US at 10pm. Meaning it would arrive in the US at about midnight (8 hours flight time minus 6 hours time difference). Again, no onward connections same day from the US, as there are virtually no departures in the 1-5am window. An alternative would be to have the aircraft sit all night in Europe for a morning departure. But that is bad aircraft utilization. The airlines can achieve the same morning departure from Europe by flying TATL eastbound overnight.