I would suggest there are many contributing factors and whether people like it or not Summer is one of the more prominent on that list, simply due to others sports being participated in by a condiserable number of 15-18 year olds, Cricket being an example.
However the biggest reason in my opinion is the amount of RL being available to a large percentage of players at these age groups....................for example............
Any 15 - 18 year old player of a good standard could be in the following position, he could train and play for his community club, Scholarship or Academy team and School,College/Uni team all in the same week whilst also being involved in Camps/Regional squads, Touring squads etc etc etc.
This requires huge dedication, commitment, time and cost at a time in their life where they also have many other significant things going on, such as the business end of their education.
Far too many players are pushed and pulled from all directions, lead to believe there is the dream at the end of it all that they hope one day will be reality (SL), but more often than not are left to drift out of the game when the next apple of some scout's eye, academy system etc comes along and is drafted in to the squad.
I have said it for years, less is often far more and until we have a proper system that can identify talent in an appropriate manner rather than en masse and tie up far to many youth players in a Pro environment of which only a very low single figure percentage will ultimately go on to achieve succcess, we will only see it go from bad to worse.
Until such time we should have a pathway that is straight forward for all to understand.......................
If you play for your Community Club you don't play in a Pro environment
If you play in a Pro environment you don't play for a Community club
Schools, Colleges and Uni Teams cannot contain players who play in a Pro environment
This would mean that the THE RFL & PRO Clubs need to identify, coach and kit out players as well as fund, manage and administer competitions for all players they want within that Pro environment and of course provide facilities and support systems to accomodate the same.
After all, it is alleged that within such an environment players thrive, develop and push on much quicker as players due to the overall higher standards of all of the above, so if that is delivered to a smaller group of players per Pro club at U16's and what is now U19's Academy and they were only participating in those competitions because we want to develop the SL players of the future not over train and over play them, it would I believe make the Community game far stronger and make the people that should be accountable for the games future, ie The RFL and Pro Clubs, accountable and as such, by putting their money where the mouth is they can prove it, either that or not run Scholarships and Academies that contain far far to many players who they want bouncing in and out of all competitions and teams when they are not playing for their "Pro Club".