I have assumed for a long time that there are differences to the nature of the Star Wars galaxy. Differences in it's physics, the characteristics of space, and the passage of time. It's long ago, and far away in a galaxy that we might as well assume is alien to our understanding. I've always thought that the vacuum of space is rough in the SW galaxy, but not as rough as in our reality. It's fantasy/fairy tale outer space. A person will need survival gear, so they've still always needed space suits, or at least a supply of oxygen.
Han and Leia walk around in an environment (space slug's mouth/throat) that can't be closed off from space, and they are only wearing respirators to breath.
Finally, it could be any combination of differences of life and environment in the SW fairy tale galaxy: space is different, humans (or what we see as humans) have different physical capability, and finally the fact that Leia is strong in the Force (and fans wanted an answer about if Leia has gained strength and training, so the movie had to find somewhere in it's story to make the point).