Wa alaikum assalam
This would imply that Prophet Noah knew exactly what lands would be engulfed by the disaster or what the extent of the disaster would be. His prayer was heard not to allow any disbelievers alive (71:26-27) and we have no way of knowing exactly how far and wide the disbelievers had spread.
Furthermore, he was simply inspired to build an ark and that is what he did (11:36-37). Migration with a view to leave the disbelievers behind was not an option in the Divine plan.
I hope this answers your question, God willing.
Regards,
Joseph