Five seasons is the magic number for a show, it is something about rerun and profit. It also has a lot to due with "scale", how much everyone is paid, after that mark is reached it, the dollars start to add up. There is also contracts, actors don't want to get tied down to a show.
As far as SG:U, they could have just put Atlantis in space, it is a space ship!
As far as SG:U, they could have just put Atlantis in space, it is a space ship!