More importantly, FNC can for the most part only get its signal to the satellite from its New York studios, while CNN can "uplink" from Washington and all sorts of other places. So when FNC had a few generator problems the night of the blackout (they ended up having to run tapes of old programs completely unrelated to the blackout for about 10-15 minutes a couple of times), people jumped to CNN and didn't jump back. (This is a problem FNC can and will fix, since theoretically anyone with a satellite truck can put a signal out and "become" the network, but FNC had some badly-designed setup with their Master Control room that prevented this for some reason. Michael, maybe you know more about this? I never really learned the engineering side of things as much as I should have.)
Also, it's August. That's traditionally the month in which nothing happens, and all the big names go on vacation. (Notice, for example, that Bill O'Reilly and Shepard Smith were gone for huge chunks of August.) Thus, fewer viewers. CNN, having no big names (heh heh), did not have the problem of viewers tuning in to watch anchor X, getting subanchor Z instead and changing the channel.