Mainly because nothing they put it in is that impressive? It makes the Hemi seem like it's down on power in comparison, I dunno. The Challenger is a fail really, it's designed to be a performance car, but it's a performance car that doesn't perform. Styling alone isn't enough to carry it, and it absolutely was meant to go head to head with the Mustang and the Camaro, only until it was obvious that it couldn't hold it's own did people start saying it's not a pony car.
I had a 09' Hemi sitting in my shop last month (truck engine out of a Aspen that I swapped that needed rebuilt.), and I passed on using it as a foundation for building my truck simply due to the lack of aftermarket support and cost.