3 (the big reason) The stairs are very narrow and EXTREMELY steep. They are small enough that I have to walk sideways down them. When you go down, you go onto a hard-wood floor. Given GSD hip/elbow problems we didn't want her dealing with the stairs esp. in light of her going down onto a slick surface. They are steep enough that she's somewhat uncomfortable with them, but she doesn't mind stairs at all in other houses.
A word of caution- If the stairs are that steep and narrow, I would reconsider using one of those rooms for the baby. My sister had a 2 story and the babies rooms were upstairs. Then she fell carrying her daughter. This nearly happened several times to all of us (even my strong & agile husband). No one cationed her against it beforehand and luckily no one was severly injured. They put their house on the market and moved to a single story home. Not suggesting that you should do that, but they had no choice as all the bedrooms were upstairs and they have twins.