[TheCoelacanth]

Sewage is far more difficult to handle than water.

You need to maintain a continuous downward slope. You are very limited in how you can have bends in pipes or two pipes join each other. You need to make sure air can get in and out of every point of the pipes, otherwise differences in air pressure will make things get stuck inside.

With pressurized water it just gets pushed wherever you route the pipes and you don't need to worry about the exact route nearly as much. Yeah, you need pumps to get the appropriate pressure on higher floors, but it's still simpler than sewage.