The one word answer: pressure. Inside a spacecraft, the pressure from the air will push outward on the door. You’d think that would be a good reason to design the door to open inward, so that the pressure from the air keeps it closed. However, as we learned the hard way with the Apollo 1 fire, if the air itself is trying to push the door closed you can’t open the door when you need to. At normal atmospheric pressure of 14.7 lbs per square inch, (101 kPa), the force on a typical door in your house measuring 3 feet by 6 feet would be over 38,000 pounds. This is so great you couldn’t open it and get out if you needed to. To make sure people can get the doors open they design spacecraft hatches to open outward so that the air pressure will help push them open in an emergency.