I think there is also an aspect of military strategy wasn't the greatest at this time in history. Battle tactics were still being developed and it was hard for those kind of things to spread. Granted, there had been some great tacticians in history up to that point (Alexander, Caesar, Hannibal etc.) but it wasn't that common.

This is touched upon in the show when King Ecbert has access to some long forgotten manuscripts (I believe from Julius Caesar) and needs Athelstan to translate, Ecbert mentions that these were the tactical works of military geniuses that were essentially lost.

As a follow up, if you like badass military strategies. check out the Battle of Alesia, Julius Caesar is outnumbered 4-to-1 in a siege against the Gauls, Caesar had a wall made around the city (which held over 80,000 soldiers plus civilians) this surrounding wall, called a circumvallation was 18 kilometers around, 4 meters high, and had four meter deep ditches on the inside to slow/prevent the Gauls from breaking through. Oh yeah, they built it in three weeks!

Towards the end of the construction the Gauls figured out exactly what was happening, they were being trapped to be killed by attrition, so they sent a group of cavalry out the unfinished section, these riders managed to escape. Caesar realized that help for the Gauls would be on its way.

So what does Caesar do? He builds another wall around the outside to protect his forces! This is called a contravallation. So now Caesar and the Roman forces are in between two sets of walls attacking a city inside, and defending themselves outside. Caesar won by defeating the outside forces of over 60,000 soldiers, causing the Gauls inside to realize that they were stuck and surrender. I know this isn't really related to Vikings, but I think of this battle anytime tactics are discussed, so freaking crazy!