I do not find the imperative in the sentence.

"Why doesn't she really study? I mean, study and not try to."
"Really study" (present indicative) here is meant to be more serious and an effort of concentration  than "try to" (present indicative).

Getting away from verbs, try the concept with nouns.

"Why don´t you buy a car, not a bicycle."

"Why don´t you stay a whole month, not just a week."