In general, the margin for error of the defensive position is much smaller and critical than for forwards.
I think that's pretty much it, an athletically talented forward can outskate his mistakes, even a fast defenseman doesn't have time to recover from his mind farts (because if you turn it over in your defensive zone it's often in the net before you can recover).
Example, if you're clearing the zone and make a bad pass, it's suddenly a play on net, if you're a forward and make a bad pass at center ice or the offensive zone, it's a rush down ice but your defensemen have a chance to cover for you.
Defensemen also have to have more subtle skills like positioning, working with their goalie not to screen them, etc. It takes time for these things to become instinctual, same with decisionmaking on offense, like when to step up and when to be conservative. For forwards, it's more about effort, hustling to get back on defense.