I don't understand why できている was used instead of できる? Answer: If you use できる instead of できている, the meaning of the sentence changes. 一人だけ記憶を保持できている means " Only you have been able to retain memories "／" Only you could memory（something） and you are retaining it even now ". Can できる in the sense "可能" be in the progressive tense at all? Answer: できるhere has not only a meaning "to have competence/ability", but also a meaning "to show the competence/ability". So you can use "できている".