Because Normans assimilated. When Norsemen came to France, they settled in to Normandy and intermarried with the locals. Successive generations were less 'Norse' and more 'Norman'; taking on bloodlines, language, and the religion of their subjects. Normans were a mixture of their Viking ancestors and the Frankish locals.

When William conquered England, Norman nobles and knights like Roger de Beaumond were given titles and founded houses such as the Earls of Leicester and Warick. Branches of William's dynasty conquered parts of Wales and founded the FitzGerald dynasty (Fitz being the standard modern spelling of the Anglo-Norman Fitz/Fiz/Filz -- from latin fils de, meaning son of). Welsh Norman knights invaded Ireland and conquered Dublin, famously becoming "more Irish than the Irish".

The DeHautevilles were lesser knights of the Normans, and went on to conquer/found the Two Kingdoms of Sicily and the Principate of Antioch. Everywhere they went they assimilated; intermarrying with locals and creating new dynasties.

The Normans never disappeared, they just became part of those they conquered (Ireland, Wales, Sicily) or combined with locals to make new strains (Normans). The 'Normans' were never cohesive a cohesive nation, just a collection of Knights who conquered and married in to Nobility and then became intertwined both in blood and title with those they conquered.

(A lot of this is coming from Lars Brownworth's book and podcast, and another book whose name escapes me; I'm at work else I'd give you direct citations).