In Delta District anyway, the clubs were given discretion on what day their U18 boys played, with the proviso that they pick a day and stick with it. I know for us in South Delta, we had the U18 home games on Sundays, as we had more available field space (more boys teams than girls), especially when the weather got crappy and everything moved to turf.
When I coached in Burnaby back in the dark ages, at U18 we played a fair number of Friday night matches.
I would guess that a big part of it is field availability..
the u18 season may be the most difficult for many reasons, including the difficult-to-predict schedules.