Very sad.
To clarify, in uncontrolled airspace it is the pilots responsibility to maintain visual separation from together aircraft, not ATCs (though often they do help). Light aircraft and helicopters are unlikely to carry any form of TCAS or traffic monitoring gizmos in their cockpits.
Easy to say "how didn't they see each other!", but I assure you it's very difficult seeing each other in the air sometimes, especially if the heli was above the Cessna being blanked by its wing. Helicopters are also buggers to see from a distance due to their size. Also closing speeds, even with small aircraft, can be remarkably hight, 250mph+ easily.
Sad to see this happen, especially as it looks like a training schools aircraft (with respect I will not name the school), so likely an instructor and student.
A sad day for aviation.