Top D-men get more ice time than forwards. That is an appeal for some.
Crosby may play 20 minutes while Doughty gets between 25 and 30.
At the minor/youth level though, they probably roll 3 lines and 6 D until there is a need for a goal or a special teams situation. This is more for AAA, AA and A teams.
Some kids want to play a certain position due to a favourite player.
Mastering backwards skating at a young age is often a big factor.