When King Edward died, the plan was that his second son would take the throne. By all indications, neither Edward nor the power structures within Wessex, wanted AEthelstan. The young AEtheling had been discarded and sent to be raised in Mercia far from the halls of power. The throne of Wessex wasn’t for him.
It was for AEthelstan’s younger half brother, AElfweard. But he died soon after Edward (under questionable circumstances) and so the path opened up for AEthelstan.