I think a better question is what he could realistically have done to prevent it. This is not my area, so I can't comment in depth, but I'd be interested to hear if it was even in the power of the French king to control William.

Edit: I mention this especially because my (perhaps mistaken) perception is that the 11th century is a moment when the French king is quite weak overall, while the Dukes of Normandy were relatively powerful. The Normans, after all, had come to rule Normandy in the 10th century because they repeatedly defeated royal armies, and were effectively bribed with the territory there. After they were granted the Duchy of Normandy, the French king more or leas attempted to subvert them culturally and dynastically, getting them to adopt Christianity and marry into French noble families. This may have ended the threat of Norman invasions, but the Normans remained more or less beyond the ability of the King of France to actually control. Now, 1066 is several generations later, but my perception has always been that the Dukes of Normandy were somewhere between "overmighty subjects" and purely nominal vassals to the King of France.

I don't have my sources on me right now (visiting my family for Christmas) but the answer is quite simple: King Philip of France was 14 years old and didn't have the power to prevent a Norman invasion of England. Philip's father, King Henry I, was one of the Conqueror's biggest rivals; during his reign he worked with Count Geoffrey "the Hammer" of Anjou to weaken the Norman Duke and together they invaded Normandy on two occasions. Luckily for Duke William, both King Henry and Count Geoffrey died in 1060, and thus the French throne passed to a boy of 8 under the regency of Count Baldwin of Flanders. Count Baldwin also happened to be Duke William's father-in-law. So when the opportunity came up in 1066, William was virtually unopposed in France and felt comfortable enough to leave his duchy to invade England.