You have to remember,
1. last year the GOP convention was after Labor Day, not before.
2. 500,000 viewers in red states were out of power.
3. The majority of people have made up their minds.
With most people in the country on a high bandwidth internet connection, it is easier to stream video and I don't think they measure that when they compile their ratings. I also don't think they are counting C-SPAN viewers.