Given that he's the "regular season" points leader, it does seem a little unfair that Biffle's championship chances aren't really getting played up by the media. That said, I think there is some basis for the belief that it's more important to have shown dominance than to have shown consistently solid results when looking ahead to the Chase.
I think the simplest way to explain that is that it's hard to fluke dominance, while the complete avoidance of really bad races generally is something of a fluke. Johnson has been the class of the field many times this year, but has also had some mechanical problems, crashes, etc. While I wouldn't call them bad luck per se, I don't think they're very predictive of what will happen going forward. Biffle, meanwhile, has had an awful lot of finishes between third and fifteenth. Importantly, he also hasn't had a truly bad finish all season; his worst result was 24th at the spring Pocono race. This is kind of the Johnson situation in reverse: his consistency is admirable, but it's also pretty fortunate that he hasn't suffered an engine failure or a race-ending crash. Had both drivers had an equal share of these types of problems, I think it's fair to say that Johnson would easily be ahead in the points after 26 races.
Being so short, the Chase rewards certain strengths and reduces the importance of others. I think there's little reason to believe that Johnson or Hendrick are legitimately more prone to crashes or mechanical failures than Biffle/Kenseth/Roush. And I think the historical record shows that a short ten-race stretch such as the Chase is likely to be won with dominance - which Johnson has already shown himself capable of achieving so far this season - rather than with consistency. So, while I greatly admire the work that Biffle and Earnhardt and Kenseth have done in performing so steadily all year, I think Johnson unfortunately has to be considered the favorite for the championship this year.