The prevalence of 'E' names comes in part from the fact that modern English has mostly dropped 'æsc', the Æ letter which is prevalent in a lot of Old English names. In modern English transliterations, the Æ is, often interchangeably, rendered as either A or E, so that Æthelred often appears as Ethelred alongside Edward or Edmund, rather than alongside Ælfred or Athelstan (which can appear Ethelstan). Of course, you could then ask 'what's with all the Æ names?'

Between the reigns of Ecgberht and Cnut, the regnal list of Wessex and then England runs: Æthelwulf, Æthelbald (predeceded by his brother Æthelstan), Æthelbeorht, Æthelred, Ælfred, Eadweard (and in Mercia his sister Æthelflæd), Eadmund, Eadred, Eadwig, Eadgar, Eadward, Æthelred, Eadmund.

As you can see, there's a definite trend or two here, although there are some cases of repetition. Eadward in particular is a popular name, with there being three King Edwards before the 13th Century 'Edward I'. So, what are the conventions?

Old English names, in particular royal names, are often kennings, compound words of two or more others, typically in the form of adjective/referential noun-noun. These are common phenomena in Old English; famous examples include "Whale-road" used as a synonym for 'sea' in Beowulf (Beowulf itself a kenning for bear or bee-wolf), or bān-loca (bone-locker) as a synonym for 'body' in medical texts. With this in mind, the Cerdicing dynasty is fond of two particular root words for its name-kennings: Æthel- and Ead-

These roots are good choices for regnal names: æthel translates as "noble", while ead- translates to "happiness" or "prosperity". Æthelwulf (noble-wolf) therefore names his sons Æthelstan (noble-stone [perhaps lit. a cognate of Latinate Peter/Petros]), Æthelbald (nobly-pale), Æthelbeorht (nobly bright/clear), Æthelred (nobly wise/informed), and the outlier Ælfred (Elf-wise/informed). Ælfred's son Eadweard (prosperity/happiness guardian) likely names his son Æthelstan after his uncle, and passes his own joy and prosperity onto Æthelstan's brothers Eadmund (prosperity/happiness - hand (lit)/ protection or security) and Eadred (prosperity/happiness - wise/informed), while Eadwig was prosperity/happiness blessed. Perhaps the nicest kenning-name, while not royal, is Eadwynn, modern Edwin, which translates to happiness-joy.