Yeah, that's about my guess, too. They had a design that worked fine for the components they had when they made it, but the next generation of components changed just enough that things went south. Maybe the cooling didn't quite work reliably anymore, or they couldn't get the support hardware on the same tiny boards...who knows. But I'm betting it's not that they just ignored the Mac Pro and never thought about revving it--it's that they tried a revision and never got it to work reliably, and they realized that they'd innovated their asses into a corner.