I'm watching them all this time. Can't say I did that last time.
I suppose that with only one major party having major debates, our attention isn't spread out so much. That may be part of it. If I'm watching two parties, I only watch about half the debates of each parties. But this year there is only one game in town, so you have six million instead of 3 million viewers watching.
Also, there's more on the internet. I actually watched the last one, in toto, on my computer. That wasn't as easy to do four years ago.
Finally, the unemployment rate is much higher than it was four years ago. This factor manifests itself two ways. First, folks are more interested now than they were back when all we could find to argue about was same-sex marriage and the like. Now, we're arguing about pocketbook issues. Much more important. Second, now that ten percent of the people are out of work, instead of five percent, you have twice as many people lying around watching TV instead of going to bed early or doing reports or preparing for the demanding day tomorrow. This also increases debate viewership.