It's a somewhat vague premise that the Norman conquest isn't "considered" French. Culturally and linguistically it represents a replacement of the Anglo-Saxon aristocracy with one speaking Norman French and practicing Norman French customs. Many generations of English monarchs spoke French as their first language. William the Conquerer never learned English. When middle english emerges centuries later, it is vastly different from anglo-saxon, in large part due to french influence.

Politically speaking, the conquest was made by the Duke of Normandy. Despite being part of the kingdom of France, Normandy operated very much as it's own independent polity, as was the case with many Duchies and Counties. In the 11th century, the King of France was in a very weak position, directly controlling only the immediate area around Paris, and exercising little influence over his more powerful vassals. This is not to say that the Lord/Vassal relationship did not exist; William himself had sought the aid of Henry I as his overlord when facing rebellion in his own land. But it was certainly not in the interest of any french monarch to have an overmighty vassal like the Duke of Normandy become king of a foreign kingdom. The invasion did not in any way represent an action of the King of France. Philip I, 14 years old in 1066, likely hoped that William would not make it back.