Why do Ragnar Lothbrok's men carry their own banners?

1) the title ealdorman or earl was only used in England by English governments. It was related to the Scandinavian word jarl, but no Vikings were ever earls, unless they were working in England for the King of England and were appointed earls by him. I don't know if you or the TV show is responsible for the use of "earls".

2) In English a heraldic emblem is called a "coat of arms". A coat of arms is always displayed on a background that has a clearly defined border, though the shape of that border can vary greatly. For example a coat of arms can be on a shield. Or a coat of arms can be on a flag, usually of the type called a (heraldic) banner. Thus it is rather inaccurate to say that they carried the banner of their lord on their shields, instead of carrying the coat of arms of their lord on their shields.

The use of a coat of arms on your surcoat, shield, banner, horse cloths, etc. was almost always to show who you were. Thus a warrior almost always used his own coat of arms if he had one. In the Later Middle Ages low ranking warriors without their own coats of arms would wear the badge (different from a coat of arms) of their lord on their clothing, and/or wear a national symbol such as the cross of St. George of England.

But Ragnar Lodbrok lived 300 years before European heraldry became commonly used.

People have been decorating their clothing, shields, flags, etc. with images and symbols for thousands of years before European heraldry was invented, so Vikings would certainly sometimes decorate their shields with symbols. But I am not expert enough to guess what those symbols stood for in real life.

Possibly the costume and prop designers for the TV show wanted to make Ragnar seem so powerful that each and every one of his warriors was a lord and the head of a group of warriors and thus used his own design instead of the design of his lord.

Or maybe they intended that each and every one of Ragnar's warriors was a delegate or draftee from the band of another lord subordinate to Ragnar and thus carried the shield emblem of his lord who was subordinate to Ragnar. Perhaps Ragnar takes one warrior from each subordinate war band each year.

Or maybe the warriors who fought for lords other than Ragnar were not important characters for the viewers to keep track of and thus carried shields with the emblems of their lords, while some of Ragnar's men were important characters in their own right and thus were given individual shield designs to help the viewers tell them apart and keep track of them.