Can someone explain to me what exactly Jankowski has done to be any more valuable or quality than a guy like Leivo?
The biggest difference between Jankowski and other prospects is the 'late-bloomer' status.
As in, with most prospects, their development curve has them reaching an impactful level around 20-22 years old or so. So you expect them to break in (if they're a first-rounder) around their draft+2 or draft+3 season at the latest, and make some noise. But Jankowski was different, because even though his talent level was admired by scouts, he was expected to take a massive amount of time from his draft to be ready for pro hockey. Not only was he days away from being in the 2013 draft, but he grew a full 6" in the two years prior to the draft. Then he grew another two inches after the draft, meaning his body was significantly underdeveloped relative to peers from his draft class and age group.
So the difference is really one of expectation. For a guy who isn't a late bloomer physically, the expectation is that he'll be in the NHL sooner, and so if he isn't, it's a concern. With Mark Jankowski, the expectation right from the start was that he'd take 5 years. That's what all the scouts said, and that's what Feaster and Weisbrod said. So there is absolutely no reason for concern in his case, because he's met expectations.