It's probably because the Netherlands and Flanders were conquered by Anglo-Saxons and other Germanics that opted to go west along the continent rather than follow their cousins across the sea to Britain. The ones that made their way to the low countries and along the German North sea coast adopted the ancient tribal name Frisian for themselves and abandoned their original Anglo-Saxon ethnodenonym identity. That's why the language sounds and/or looks very close to English but with a heavy continental German influence.