Because gravity is a bitch when making squishy things. They like to collapse under their own weight before they can harden. Imagine a pipe that is rigid when fluid is flowing though it, but collapses and sticks together when none is present. There would be a massive amount of difficult in attempting to print/build said pipe under pressure.

In zero gravity you don't have to worry about pressurizing it, but you do exchange a new set of difficulties.