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".