Mind you, I am not an architect, but any blue prints I've ever seen have also had the entrance at the bottom edge of the paper.
So I'm guessing it is more a general design informal-standard, advocated by (as mentioend above) that if you were standing there, you would be looking forward to the stuff and the paper would be picked up and turned whichever way the building was to be situated. Now put that on a map where you start the regular design way and then after the fact have to stick on compass points to the page ...