To give the fresh air and the humidity as much access as possible to the whole surface of the egg shell. I hatch in cartons, and I cut out the bottom of the egg 'cup' as well as cutting down the sides of the carton as well. You just need enough carton left to hold the egg in place, adn you don't want a chick pipping below the top of the carton and maybe getting stuck or not being able to get its beak out to the fresh air...
Carton hatching is great - I love it. It's tidy, and it's easy to keep track of which egg is which, as they never roll around...