Apple Doc:
When this property is set to YES and a valid camera is associated with the map, the camera’s pitch angle is used to tilt the plane of the map.
I don't see you setting any camera.
A sample camera implementation:
MKMapCamera *newCamera = [[mapView camera] copy];
[newCamera setPitch:45.0];
[newCamera setHeading:90.0];
[newCamera setAltitude:500.0];
[mapView setCamera:newCamera animated:YES];