There was no concept of a nation-state until the 1800s. With around three roughly equally powerful states covering most of modern England during the heptarchy (Northumbria, Wessex and Mercia)--which were themselves more focussed on protecting themselves from being invaded than actually expanding--there was no easily achieved hegemony before the foundation of England.

Also, between ~400AD (the end of Roman rule in Britain) and ~830AD (the hegemony of Wessex established over England) Britain was flooded with invaders and settlers, the Angles, Saxons and Jutes only came to Britain during this time. The Anglo-Saxon states of England only existed for around 400 years maximum. Comparing this 'lack of unification' with other regions of Europe show similar fragmented states (cf. Iberia).

But your question is somewhat leading, it implies that England was an entity which needed to be unified when in fact England as a state exists as a fluke of history, similar to France or Switzerland. These countries aren't nation-states like Germany or Italy. There was no concept of "unifying England" because "England" didn't exist, it never had.

Essentially, England was only "unified" (and bear in mind that at this period, Cornwall was not considered part of England) because of the Viking invasions. No English King ever united England (though some kings, such as Offa, achieved a prestigious position considered above other Kings) because there were a number of unique kingdoms with similar strength. It only happened after the Vikings conquered every country besides Wessex; when the House of Wessex later took over Danelaw, they didn't give the new territories back of course, but kept them, and so Wessex became England. It should also be said that what we consider England is England because that's what was united. Anglo-Saxons living in the south of what is now Scotland did not become part of it, while Brythonic speakers in Cornwall did, and so Cornwall is considered part of England and southern Scotland is not.