Given the reference to all of the kingdoms but Wessex being under Danish rule I assume you are referring to the Great Heathen army not earlier Viking raids. So, I'm going to focus on that time period.

England was very divided at that point. Obviously it had many kingdoms so that suggests division in and of itself. But there had been times when the Danish would have encountered a relatively united south under Mercia or a relatively united Northumbria. But by the 860s many Anglo-Saxon kingdoms had seen years of internal strife and intra-kingdom warfare. Northumbria, for example, had seen years and years of dynastic struggles. So, that by the time the Danes got there the nobility had been thinned out through warfare and not totally ready to unite effectively against Danish incursions. The Vikings were pretty easily able to establish themselves in Northumbria giving lip service to a local king of their choosing. That gave them a pretty secure base to attack the rest of the island.

The Danes coming against a united England may have faced a much greater task. (Although not necessarily since a united Denmark was able to conquer a mostly united England under Sweyn Forkbeard and his son Cnut.)

Beyond that I think the better technology is certainly arguable. The English didn't have anything similar to the longboats. And the longboats didn't just allow the Danes to sail to England. It allowed them to navigate rivers. That made lightening quick strikes possible. At least for the areas accessible by river. By the time of the Great Heathen Army they also had weaponry that was comparable from years of taking weaponry as booty, working as mercenaries for the Franks or even further afield in Byzantium, getting weaponry as homage.

More money is also somewhat arguable at least for some of the kingdoms. Wessex (and especially Kent) was quite well off. But not all of the kingdoms were. And the Danes had received extensive danegeld from Anglo-Saxon kingdoms and from the Franks. Some of them had also fought as mercenaries where they were well paid. They also had permanent bases in Ireland. So, these weren't just pirates anymore. They did have access to funds and they had access to settled land close by.

The more people is true but it isn't on the scale you might think. Those English actually trained to fight in a meaningful way were the minority of the population. But the Danes were taking men from battles in Ireland, France and Scandinavia. They had a large force by 865 and, at first, they only grew larger as more Norse stragglers from various locales came to join them. And locals would join them, as well.

The elite of English society would be highly skilled train warriors. But most of the peasantry weren't and the fyrd system enacted by Alfred wasn't in existence yet. So, a peasant militia wasn't useless but against the battle hardened Danes they wouldn't be particularly effective. Trained warrior v. trained warrior you have a much more narrow ratio. And the Danes would have the advantage in some battles on that score.

Overall I think you shouldn't necessarily think of these people coming directly from Scandinavia. Sure, the English had more people, money, and land than most back home in Denmark or Norway. But by the time of the Great Heathen army they had been fighting all over Europe. They had established permanent bases in Ireland and, later, York. They had resources and they were battle hardened.