Because that's what the manufacturers chose to promote.
Without getting into tedious detail, the whole key to a factory race program is the advertising budget. In the recent past, the Cobalt ad group decided to spend some of their ad money on racing and funded the program. Not Camaro, not Monte Carlo, not Malibu. I assume it works the same way at Mopar.
Look at it this way, a Camaro or Challenger pretty much sells itself. The other cars might need a bit of a nudge to get placed in our radar screen when it comes to buying a new car.
Yes, there's a question of which body will offer the most performance and each manufacturer's field rep will often lead the discussion towards one particular platform over another.
But since we landed in Neverland a few years back I'm not sure it really matters as the NHRA seems willing to allow almost any customization to make even a Avalanche competitive if that's what GM really wanted. Or Ford, Mopar, or anyone else willing to step up and spend.