The Vikings absolutely did use spears: sources such as The Anglo-Saxon Chronicle, the Battle of Brunanburh, the epic Battle of Maldon and Asser's Vita Ælfredi all make reference to the Danes adopting a shield-wall and fighting with spears. But they also did use axes for precisely the reason that /u/Platypuskeeper suggests: they're commonplace, everyday tools that everyone would likely have access to, but could also be used conveniently to kill somebody. This would make them popular for maritime raiders in particular who would want to be quick and mobile, and would be hoping not to need the equipment to fight an actual battle at all.