I don’t handle the Young Writer’s Program, but as a moderator of the regular site’s Discourse I have some insight here.
NaNoWriMo, or better known as National Novel Writing Month is a series of writing events that happen in April, July, and November of each year. It’s a self-challenge, and one thing the users ask for a lot is a way to write their stories directly on the site/forums so that they can easily and automatically update their progress.
The YWP program does this because, as the name implies, it’s used by kids and teens, sometimes independently, and sometimes as part of their school curriculum. Most users know this function exists on the YWP site and regularly ask why it doesn’t on the regular site.
For the record, I actually do sometimes write on the forums in self-PM, and later have a tool that will sync the Markdown to a Scrivener project, so I get why users want that functionality.