As I understand it, there are two reasons for the lack of daytime US -> EU flights. Firstly, there's the effect of curfews at both ends combined with the timezone and flight length, which means there's only a small window in which you can leave the US once the airport opens but still make it to Europe before they close for the night. Secondly, there's the way the tracks across the North Atlantic work, which means that during daylight hours all tracks are westbound. Eastbound daylight flights therefore have to take a less optimal route, which adds to cost, and can mean longer flights which makes the curfew-to-curfew thing even harder to meet.