I don't think so. The majority of the fanbase is pretty much represented in the top ten, except for Smoke.
The numbers weren't this bad last year with Jr not being in the Chase. He has the largest fanbase of all I think.
I think it's that they NEVER promote the race on NBC/TNT and their coverage has sucked with too many commercials. That's the fault of Nascar of course for charging the networks too much to begin with......Brains and Greed.
Then there's the inconsistent rules and the changes in the cars this year which has, at times, made for crappy racing since the teams can't get a handle on the setups because of the lousy tires, spoilers, camber, etc.
How many fans would need to defect to drop ratings 10 to 15%? And remember Jr and Flash still had plenty of coverage because they WEREN'T in the chase...