If Don Walsh and Jacques Piccard were able to travel to the deepest part of the ocean why cant we now?

In 1960, Jacques Piccard and Don Walsh traveled to the deepest point in the ocean (Challenger Deep of the Marianas Trench, which is about 35,800 feet deep). If they did it then, why cant we do it now? Scientist say that the pressure would kill us, but if they could do it in the 1960s why cant we do it today?