The TV rating is developed from the absolute number of households with TVs tuned in compared to the total number of US households. The importance of relative comparison cannot be overestimated, as population steadily increases each year. Alternatives to TV also have proliferated. Specifically, in the 2008 campaign, it was easy for someone to watch the convention on their computer -- live or after-the-fact.
Data used in this chart is available: Historical TV Ratings For Republican Conventions .