The youtube video currently has ~1.4 million views. Colbert averages 2 to 3 million television viewers per night, plus some number of youtube views that I haven't looked up the stats for.

That is, this interview has been seen by fewer people than it would have, had it been on television.

Let's give it some time. It hasn't been up that long, and it's already gone up to 1.9MM views in the 2 hours that have elapsed since you posted this.