According to historical story/myth, Tale of Ragnar's sons (Old Norse: Ragnarssona þáttr), Bjorn's mother was Aslaug. Yet in the 2013 show Vikings, Bjorn's mother was Lagertha. Why was this change made? Have the directors or writers explained the difference?
Altought they didn't give any reasons why this change was made, there's the Lagertha reason. They needed a strong female character. This is one of VIkings's many caracteristic. You're a Viking; you're strong; you fight.
It was more of a need for a character that the watchers would love and get attached to than anything to do with the tale of Ragnar's sons.