The Nielson company that does the ratings didn't include PVR stats for a long time. Likely something technical. Now I believe they do PVR stats as well. But many people (myself included) believe the networks place far too much confidence in the Nielsons.

The bigger problem however is that advertisers (also) look at the same ratings and won't be inclined to advertise on shows with low numbers. That's why it doesn't matter how good a show is nor how many people watch it or its commercials. It's all about what numbers the people with the magic boxes report.

Thankfully there are specialty cable channels where the bean counters think differently than their counterparts at the broadcast networks, which has lead to the creation, and continuation, of a number of fantastic series which would not have lasted a half season on broadcast, if they were picked up at all.