Because Blackbeard has split personalities, and one personality is much stronger than the other. It's how to explain how Blackbeard "Never sleeps", because when one is asleep, the other is awake.
How many times do we see Blackbeard getting hit by attacks, then dropping to the ground and writhing in pain, that most other characters would shrug off? It happens quite often in the manga. But then during Marineford, Blackbeard takes a quake blast to the face from Whitebeard, and then afterwards takes a shockwave from Sengoku and he tanks both of them with minimal problems. How else do you explain how something like a jet Bazooka can blast him into a wall, dropping him, yet a Shockwave from Sengoku just drops him to one knee. I think when his main personality hits its pain threshold, and starts to feel extremely threatened, that's when the second personality shows up. And I know, the Blackbeard having multiple personalities theory has been around for years, as I made one 5-6 years ago myself. To me it just seems to make the most sense at the moment.