3 excuses.
BIG EXCUSE X: "Our target audience was still doing their homework by the time the show aired, but by next week, they'll have dropped out of school, so we should see an uptick."
BIG EXCUSE X+1: "People need to get used to the show being on Tuesdays, it may take a few episodes. The SGU spots where running during the SG-1 marathon, but cool people don't watch that show."
BIG EXCUSE X+2: "The marketing campaign was missed by cool people that have lives, but now that the kids are back in school and parents are home from vacation, we'll start seeing an uptick in the ratings."
... ok a few more:
BIG EXCUSE X+3: "As with the software industry, we're seing that many people are pirating the episodes on bit torrent sites .. if it wasn't for the pirates, then people will have CERTAINLY watched the show live, just like software pirates would certainly PURCHASE the software."
BIG EXCUSE X+4: "Oh cool people are recording/PVR'ing (?)* the show to watch it later."
* I've never even seen a PVR thingy .. but doesn't PVR'ing contribute to ratings anwyays?