As the league smartens up and organizations look to maximize the long-term output of the talent they select at the top of the draft, older players tend to lose their footing and slide down the draft's slippery slope. There are exceptions, but going younger offers more of an opportunity to grow with a given team, while a player works towards his physical prime. It's simply the way the league is trending, as illustrated by Nylon Calculus' Jacob Rosen prior to last year's draft:
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This coming draft class looks to be on a similar trajectory.