I think the solid axle has some to do with it also. I know when I went to bg last year the cummins killer didn't do so hot. But that truck has had a few good pulls and is set up nicely.
A p pump cummins gets the trickle down effect from tractor pulling. And common rail diesels are just now catching up in the fuel injection rate capabilities, at the expense of your wallet. Also, in the cummins world, there are a few shops that will sell you turn key motors that will make enough power to win your class at any event, all you have to do is write the check.(big one)