A "fully alive" Marine can pilot a Centurion, while only a "mostly dead" marine can run a Dreadnaught. I imagine that given the option, a given marine would prefer to pilot a Centurion.
I agree that GK are pretty much the army in which the most special of the most special of the most special of the most special... and so on... are put. If you look into their fluff, they should probably run out of Psycannons before a 2000 point army is properly constructed.
In terms of sales, everyone already has a Dreadnaught, because Giant Robots are Phunn. That's right. So much fun I forgot how to spell. I wish IG had Dreads, instead of Sentinels.
But I digress. Since everyone and their brother has Dreadnaughts, and Marines, and Terminators, if you want to sell models to long time players, you have to come up with something new. If you don't want people to just convert old stuff... like Death Company into BA Vanguard Vets... you have to make a new "Chasis" to work with. The widest gap is between "Tactical Dreadnaught Armour" [Termies] and full on Dreadnaughts, so that's the easiest place to squeeze a new model into. Sure, there's space between Power Armour and Termies, but it's kind of filled by Artificer / Techmarine Armour.
In terms of fluff... I think that when you have such well established concepts of armour in the SM line, anything you do is going to feel like shoehorning these guys in.
"Yeah, we already established that Terminator armour is the best armour in the galaxy, but we need a new model. Uhhh... lets make an exoskeleton that goes over power armour, to be about as tough as a terminator but not so tough as to be a vehicle. Let's make a miniature Dreadknight. Perfect! Why did they just suddenly appear out of nowhere? They've always been there. But they're super rare, right? So we're just finding out about them... uh... even though the well established Chapter Masters should definitely have known about them before... and maybe talked about them?"
"Shut up Steve!"