There are few reasons for this. One is that they had a lot of smaller ships that were more versatile and in general better than European ships for purposes of raiding different lands. Moreover, Vikings also received more training in combat since a young age compared to certain armies that could be found in Europe at the time thus giving them an advantage.
Vikings were also notorious and were feared throughout Europe as savage and ruthless killers. They were also able to exploit certain time points in European history where internal struggles between leaders led their nations to be weakened which enabled easier conquests for the Vikings.