Because they are the weakest points. There's already a big crack, it doesn't take much to make the crack expand & contract. Just like a small crack in your windshield, it starts out small, but over time the vibration, expanding & contracting it starts to grow. 1 day it's had enough a bang your windshield is a spider webbed mess.
Most earthquakes happen at tectonic plate boundaries because at these spots plates collide and slide past one another creating great instability. This explains why most earthquakes occur around the edge of the Pacific Plate in New Zealand, Japan and the Americas.