How can boxers usually survive massive force to their heads, but natasha richardson dies from a fall?

Not everyone. Not all boxers actually survive! a lot of them if survive end up having traumatic brain injuries and posttraumatic brain syndrome. Another thing you need to remember, they have been trained to receive blows in their head, so they may be somewhat more resistant than poor natasha.
Boxing. Boxers do suffer brain injuries, though, as mentioned, they are usually of delayed encephalopathy following many repeated blows to the head. However, I have taken care of a boxer, who, minutes after the last notable blow to the head, went down and never got back up. He was dead a few hours later, in spite of aggressive surgical intervention. He actually had a fracture that tore a major vein.