Why did the unified kingdom founded by the Wessex dynasty get the name of England?

Because of how the country of England was formed. A great book to read on this is called Mercia and the making of England. I'll break it down into a couple of steps that shows how England came to be. In the mid-to-late 9th Century, what would become England was broken up into the four West Saxon Kingdoms of Northumbria, East Anglia, and the powerful kingdoms of Wessex and Mercia. The smaller kingdoms that included the kingdoms of Kent/Cent, Bernicia, and others had mostly been rolled into one of the four. While all Saxon kingdoms, these kingdoms bickered constantly with each other and internally. The Danish (Viking) invasions happened first in Northumbria, which was in the middle of a civil war, and then escalated south until the only kingdom that remained with a king was Wessex, and at one point it consisted of one swamp. Over the years, Alfred worked to secure his kingdom and ally himself with what remained of Mercia. Alfred had a dream of a unified Saxon kingdom of all the Angle-land. As Wessex (and Mercia under Alfred's daughter) grew more powerful, things got complicated. Mercia needed Wessex's support to keep the powerful northern Dane's at bay, but they were a proud and independent people who didn't like the idea of being ruled by Wessex. Alfred's marriage to a Mercian and Æthelflæd's marriage to the most powerful lord of Mercia helped smooth things over. By the time the Danes were pushed out of East Anglia, the East Anglians had less resistance to being ruled by Wessex. They had been ruled by Dane's for so long their ruling families were completely destroyed or in exile abroad. What's more, they were small and weak where Wessex under Edward was powerful. Things got even more sticky when Æthelflæd died. The Mercian's didn't have a clear candidate for the throne and were still threatened on their northern border but at the same time weren't ready to just accept a Wessex king or being absorbed into Wessex. Unlike East Anglia, they weren't small or weak enough to be bullied by Wessex. Wessex needed them almost as much as they needed Wessex. Things are a little murky in the historical record at this point but it appears that Edward was given the throne of Mercia on the condition that of his two sons, his oldest son Æthelstan, who was raised in Mercia by Æthelflæd, would become king of Mercia, and his younger son Ælfweard, by his second wife (or first wife, depending on the source) would become king of Wessex. This allowed the two kingdoms to remain allied with a strong familial bond keeping them together while at the same time allowing them to retain their separate identities. After Edward's death in 924, it appeared that the plan was in place, with Æthelstan crowned as king in Mercia. Unfortunately (or fortunately for England) Ælfweard died only 16 days after his father. A year later after some resistance, Æthelstan was crowned king of Wessex and Mercia near the border of both countries. Symbolically, this was very important. To the Mercian's Æthelstan was a Mercian king and it allowed them to keep their pride and national identity. The same could be said of Wessex; as the eldest son of the former king of Wessex, he was considered West Saxon and they accepted him as their king. Æthelstan's conquest of the final remaining kingdom in Northern England and even the vassalage of Scotland after the battle of Brunanburh, was just that, a conquest. A lot of little battles and marriages and diplomatic attempts to unit the kingdoms lead up to that. The threat from Scandinavia was still very real at that time and he was severely weakened from the battle. By declaring himself king of the land of the Angles he alleviated the idea of conqueror somewhat. Instead he was a liberator and uniter of the Saxons. He also inherited the vassalage of the Welsh from his father and aunt. It's important to remember that before this time the kingdoms still weren't completely united and calling it Angle-land was meant to unit them. Annexing the conquered kingdoms into Wessex would have alienated Mercia and made combining them into one nation much more difficult. tl;dr the ruling nobles of Mercia and Wessex combined militarily and by marriage to defend their lands and later push out the Scandinavian invaders. They then conquered the other kingdoms together and together formed a new nation. Incidentally, they lost the kingdom to Cnut the Great and while they later regained it, they lost it again to the Normans in 1066. The Normans were of course, Northman and Scandinavian themselves. The Dragon of the House of Wessex still displays prominently in England. The national identities of the Saxon kingdoms were basically completely wiped out in 1066. The 100 years war led to unprecedented nationalism in both England and France.