How would you explain these 2 example sentences in terms of choice of aspect?
Я не позвонила потому что не смогла.
vs
Я не звонила потому что не смогла.
?

Никак не объясню. Оба варианта допустимы.

They both perfectly fit into the basic concept of perfective and imperfective verbs, no matter negated or not. Tell me how you understand those concepts and I'll tell you how to treat both of the examples in terms of those basic concepts.