There's some debate in the Vikings fandom about who the main character of the series really is. For the first few seasons, it seemed obvious that Ragnar Lothbrok was the central figure of the show, but over time (and especially after his death in season four), the 'main character' title was up for grabs.
The main debate seems to center on Ragar, his sons, and Lagertha as the major players - but it's Ragnar's eldest son who may have the biggest claim to the protagonist title. Bjorn Ironside is, after all, a major character for almost the entire series (even if he doesn't survive to the finale), and a fan-favorite, too.
One Of The Longest-Running Characters
One of the biggest factors in deciding the main character of the show has to be how long they have been a part of it - and Bjorn has been in more episodes than almost anyone else. While adult Bjorn appears in 67 episodes (according to IMDb), with only Floki (68) and Lagertha (71) in more - his character (young and adult) appears in 76 (according to the Vikings wiki). While the other Sons of Ragnar only come into play in the second half of the series, and Ragnar dies in the fourth season, Bjorn is there from start to finish.
Involved In Every Part Of The Show
Bjorn isn't just involved in every season of the show, he's also involved in (almost) every element of it. He raids in England, Paris, and even beyond. He creates alliances, explores, raids, leads armies, battles in civil wars, and is connected to essentially every conflict in the series.
The only part of the series that he is not involved in in some way is the settlement in Iceland, and Ubbe's final journey to North America.
Connected To Every Character
Bjorn is also connected to almost every other major character in the series - either as an ally, an enemy, or as family. He even often acts as an audience surrogate, meeting characters for the first time along with the viewers, and showing fans how to react. This is especially true when he first meets Floki as a child, or when he is traveling in the Mediterranean as an adult.
A Stand In For Ragnar In Later Seasons
In addition to occasionally acting as an audience surrogate, in many ways he acts as a replacement to Ragnar after the original central character's death. The two clearly look extremely alike (and were presumably intentionally cast to do so), and have a lot in common - their desire to explore, to understand, and their leadership abilities (and reticence to do so) in particular.
Travels More Than Most Characters
While Vikings is, in part, about the politics and life of the Vikings in Norway, it is primarily about their explorations. This is seen in everything from Ragnar's original desire to reach England to Ivar's fleeing to the Rus, to Floki and Ubbe's travels to Iceland and North America. But Bjorn may be the most driven by exploration, and travels more than most (across Norway, England, , and beyond, even to Northern Africa). For a series about Viking exploration, it makes sense that the Viking who does the most exploring is the main character.
Given His Own Name
It's a smaller detail, but of all the major characters, very few get a name beyond their given name, or called more than one name - and those that do are the biggest characters.
Ragnar Lothbrok, of course, Ivar The Boneless, and Bjorn Ironside. And of these three, only Bjorn actually earns his name through his fighting prowess - suggesting that his name (and by extension, the character) is the most important.
His Tomb & Legacy
The marriages and funerals on Vikings are often lavish, impressive events - but none more so than Bjorn's. Of all the Vikings, he is also shown as the most important, as he is the only one who is given not just a burial mound, or a farewell in a flaming boat, but a grand tomb that includes a statue of him in the center. This even comes into play in the episodes after his death.
Connection To Ragnar & Lagertha
For many, the real 'main character' debate is between Ragnar and Lagertha - but Bjorn represents the connection between these two characters, and as such, may be more central than either of them individually. He has traits from both his parents, and his story carries on their legacy and that of their family.
Fan-Favorite Character
There is no escaping that Bjorn is easily one of the fans' favorite characters - and possibly even more than Ragnar, who definitely made worse decisions than his eldest son did. Being beloved by fans does have an impact when it comes to arguing for a character's place as the protagonist, and Bjorn is a clear hero, from start to finish. As a ruler, a son, and even a husband, he always strives to do what is right, and his death cements his place as the hero of this story.
Becomes King Of Norway (Sort Of)
ittedly, Vikings is far from historically accurate, and Bjorn doesn't technically get voted the first King of Norway (thanks to some manipulations by Harald), but as Gunnhilde says after his death, he was the true King. His ability to call all the other Vikings together to save Norway clearly puts him as the most powerful character in the show, and his impact on the lives of his people puts him as the central one.