I have been flying TATL since the early 60's and overnight flights have always been the norm.

This is primarily because the overnight schedule provides for more efficient aircraft usage, allowing airlines to leave the East Cost in the evening, arrive in Europe in the morning and then shortly thereafter departing back for the East Coast, with arrivals that allow for a short turnaround.

Airlines also believe that most business flyers don't want to take daytime flights eastbound because this "burns" an entire day.

As others have stated, there are a handful of daytime departures, mostly out of JFK, but well over of 95% of eastbound TATL involve overnight flights, and this has been the case since airlines started flying these routes non-stop in the 50's.