Ok first and foremost, Monday Nights haven't always been the night for wrestling. In the old days, it was Saturday Nights that everyone would watch wrestling on. In fact, Monday Nights didn't become the night for wrestling until 1993, when Raw first debuted on USA Network.
Secondly, pre-taped shows aren't always a bad thing. Last Monday's Raw, for example, was pre-taped in England and flowed really well for a Raw. And in the 90s, before the Austin Era, Raw had always been pre-recorded and continued to get decent ratings.
As for the main topic, I would have to say it's simply because of the network which has always been the case for smackdown. A lot of people either don't get the channel, or are just too lazy to take time to look and assume that just because a station changed from UPN to MyNetwork that they don't get it.
Hopefully, when Smackdown moves to SyFy, we will be seeing the ratings increase. Which is going to be really cool as then I can watch both the Ultimate Gamer AND WWE on the same channel.