1. first they were lucky it wasn't a car bomb
2. the whole royal family was on the same bus in the same city at the same time which means that if it was a car bomb the WHOLE royal family would have been killed.
3. Some professionals say that you can't foresee this and that it didn't happen before but Holland did have some violent attacks on important people about which everybody at the time said that it wasn't possible in Holland and that this was a first (Filmmaker Van Gogh and a populist politician). How many attacks should they have on important people to realise that violence against important people is more possible than before.
4. The national royal holiday is a festivity but if you look at this picture than you will see that the protection is minimal and that they are very very lucky that this was just a lunatic suicide attack by a loner who didn't wanted to be sure that he would go into history as the guy who killed or wounded the whole Dutch royal family. He could pass two police agents who didn't shoot, he could pass a some other checkpoint without being hindered (no police men) and than he could just ride with high speed into the crowd (no policemen before the last checkpoint) to get as near as possible to the bus with the royal family.
5. They were having too much fun and too less security. Securitypeople should be more paranoid and other people should have more confidence in their advice.
last thing, this is the advantage of following such important 'transports' with a helicopter in the air. He should have seen that there was a car riding where it shouldn't at high speed and could have sent out instructions to stop the car at all costs or put more special police forces around the bus (or divert the bus) and get the royal family in safety (was there a protective car riding after the bus in case ?)