Same reasons for the faillure he ends up having everywhere, he can't work with big egos. Interestingly enough many of the same players he was succesful with at Ajax in the 90's were tired of him in 2000-2002 when he failed to qualify with Oranje, because they got older and got a mind of their own. Van Gaal isn't fun to work with and he's a communicative nightmare and a megalomaniac.
National team managers need to be extremely good people managers, more so even than club managers. It's all about keeping the players happy, if you manage to do that then you can get pretty far. Van Marwijk did a really good job at that in 2010, very calm guy, consistent, provided alot of clarity. His tactics however were bad, or well that's a matter of taste, but they were different from how the Dutch team used to play. But this Euros Van Marwijk made a bunch of mistakes that I won't get into.
Van Gaal, an awful people manager. With all the tension already there from the past tournament, there was alot of conflict between the players, he's not the type of person who can fix that all. Not to mention the stuff he pulled with Ajax this season was a disgrace. He lost alot of respect in my country, he's going to get very little credit from the people and the media. His strengths are his tactics, he will at least attempt to make us play more attractive football again. But that's not something that really comes into effect the next 2 years untill the actual tournament. With all those qualification matches and friendlies when they're just together for a few days, you don't have alot of time to mould the team, it's all about keeping the players happy. And Van Gaal is always on a collision course.