I believe its more to do with the priorities of our Karachiite students. I disagree with the notion that the standard of education in Karachi is low. I believe Karachiites prioritze professional private jobs more than that of civil servant job.
Also there is an ethnicity bias among Karachiites that feudal class gets an unfair advantage over urban Karachiites. I am not sure whether this fear is founded on some myth or crass.
The standard of Karachiite is better than that of Lahore and Islamabad considering as citizens of a financial hub we have better exposure to the working of multinational, multicultural, multi ethnic corporate sector that thrives on the talent churned by our gratified educational system.
Case in point; Ned, Aga Khan, IBA, University of Karachi, Szabist, TIP are world class insitutions of Karachi.