why isn't King Athelstan a more known/celebrated in English history?

I think the main reason for Athelstan's apparent obscurity in English history is the sheer complexity and substantial difference in the time periods that individual's might be looking at when studying Athelstan. Whilst Athelstan's actual history is not terribly complex, the subsequent monarchs in English history are a bit more difficult to comprehend for the casual observer, certainly more so when compared to the likes of the more simplistic Norman kings.

Much of Anglo-Saxon history is neglected like Athelstan who won eastern England back from the Scandinavians due to the French-Norman kings and nobility who ruled England (1070-1509). For that long 500 year period Anglo-Saxon history was suppressed, the new kings and ruling class didn't want the prior English (Anglo-Saxon) ruling class/ kings to be remembered, and it kind of stuck through modern times.

I think the tale of King Athelstan gets overshadowed by other historical figures,

Poor press. In a way, though, he is sorta remembered in perpetuity, thanks to a series of fantasy books. Aethelstan was popularly known as the Lord of the Rings, for his propensity to give out rings to his favored retainers.