Why thrombolytic therapy is contraindicated in aortic dissection?

Death: In an aortic dissection the aorta is splitting between its layers and is at risk for rupture. It requires a surgical repair. Thrombolytics break down blood clots and any holes in the artery that are being plugged by clot will be opened again potentially worsening the dissection or allowing it to rupture. This would result in a rapid death. Thrombolytics would also prevent the needed surgery.

Bleeding risk: Because doctors tries to balance the possible benefits to the risk of harm. And here with aortic dissection, there is an increased risk of bleeding that exceed the desired benefit, which makes thrombolytic contraindicated in those patients.