The thing is, if you only ever work on writing something occasionally and in small doses, sooner or later you're bound to forget the plot you've built so far, the details you've put in, the hooks for the sub-plots which were supposed to develop down the road and pretty much the rest of it. You also lose the sense of the pacing your story had up to that point, and likely have altered the way you write (because years are plenty of time for your style and focus to change)So he's young and has a long time to finish this.
Even with the best intentions of continuing, you can find yourself picking up the half-finished work, struggling to remember where the heck was any of that supposed to go, and concluding that you'd have to re-read the whole thing just to get back your bearings, and you simply don't have enough free time for that. Especially as years go by and you've gained new interests, new obligations, and way more things to take care of than you had when you're just out of school.