I think a lot of it has to do with the channels in which they are shown in America. USA network has hosted Raw for most all of the time it has been a show and is okay with it running over and other things. Smackdown is on Syfy which has another show that they feel has good ratings that comes on directly after Smackdown. If Smackdown was to be live then it could potentially run over and affect the show after it and it is possible that Syfy does not want that. I have no way to know that this is true, it is just what I assume. I know that they cut Smackdown to fit into the 2 hour segment so it makes sense.