Thanks, I'm good on Normandy once established but not on what was there beforehand.
IIRC Neustria was a pretty poor part of Francia, and Viking devastation would only exacerbate (sp?) that. And since Anglo-Saxon England was one of the richest kingdoms in Europe around this time (possibly a bit later it achieved that status), it would still attract Viking raiders, who would rather plunder rich England instead of settling in poor Neustria.
Another possibility, is that because the Danes were the ones raiding Francia -- Rollo was Danish -- maybe the Norwegians would take the place of Danes raiding England ITTL, so a more northern Norwegianlaw would be established, making the British Isles more Anglo-Norwegian instead of Anglo-Danish.