probably a combination of both (and maybe other reasons as well). They barely did it on the indoor shows, so the early days were not the only reason. It had something to do with the outdoor circumstances as well (bigger audiences, daylight)
yes, there were a few shows that started in the dark, but I think at least 80% of the (European) shows started in daylight/twilight. Keeping the same setlist sequence for those few shows that started in the dark probably just was a matter of convenience.