I think it's because of economics.
Used to be the teams could pretty easily be divided into the haves and the have nots. Now, there are three...the Haves (Roush, Hendrick, DEI, Gibbs, Penske etc), the Have Nots (affectionately known as field fillers), and the Used To Haves But Ain't Got No Mores (Davis, any Bodine, Petty, Woods, etc). Even Roush is having trouble getting all his teams sponsored. Go figure, his fingerprints are on every dang Ford in the field now.
It's a side effect of the way racing is televised and covered nowadays. We all know who's going to get the camera time regardless of where he is running. Each of us can name the 5 to 8 drivers who will be shown no matter what. Who will be interviewed. It's a function of the new NASCAR. We must at all costs build, foster, create, or manufacture superstars to be the face of the sport. Happens in other sports too. Jason Seahorn hasn't dona a thing of significance on a football field in three years, but he's still quite visible in the marketing. Tim Brown, same story. NBC and FOX are simply fulfilling their purpose...to cram a handful of drivers down the throats of the viewers, regardless of how bad their asses are getting kicked on the track on a given Sunday.
I bet money that Tony Stewart is more prominently featured as a "former champion" than Matt Kenseth is as "defending champion". This has some to do with the fact that Matt Kenseth has the personality and charisma of a pineapple. He's just not comfortable in that role. However, going hand in hand with that, Tony gets more face time during the race, the car is shown more, etc etc. Nature of the beast and Marketing 101.
NASCAR survived for a decade on Waltrip's mouth, Earnhardt's smirk, Elliott's drawl, and Petty's teeth. In the 90s it was Earnhardt's legacy, Gordon's mug, and Martin's grit. Now it's a new era (or so they continually tell us) and this calls for a new strategy. Junior's charisma, Tony's playfulness, Harvick's attitude, and Newman's confidence are the standard bearers. No room for Matt's competence in that equation.
But I could be wrong...