Yeah, probably because they're worse at hockey, generally speaking. It's no surprise that the lanky kid who couldn't skate well that you drafted in R1 took a dozen years to develop into something useful.
If I ran a team I would draft only forwards, convert some of them based on things like them having a harder slapshot or slower foot speed (analogize to a pitcher having a plus-plus FB but not enough command to stick as a SP), and we would f***ing light it up. It would be like a constant power play with 6 little Torey Krugs on my blue line.