Reasons behind that genius substitution are simple. First of all, Cillessen has never saved a penalty in his career. 0/16 if I remember correctly, 2/20 for Krul. You could say the difference is small, but you definitely have much more confidence when you have saved at least a few rather than not a single one. Secondly, Krul is 2 inches taller than Cillessen. Kind of self explainatory, bigger wing span, easier to save. Even the ones he didn't manage to save were pretty close. Finally, EPL experience > Eredivisie experience. Although this season was not the best for Krul, he still played against some best attackers in the world, meanwhile Cillessen's biggest challenges this season were those 3 Champions League group matches.
Generally, it's just a brilliant move from the whole team. LvG clearly had this idea long time ago, he had the balls to pull it off, because no one in the WC history has ever done this and now we can say he nailed it. I've read that it was actually goalkeeper's coach who gave the instructions to Van Gaal, there were a couple of seconds when broadcasters showed us Krul and GK's coach warming up prior to penalty shoot-out and it was clear already that this substitution is not improvised.