I see a couple of different explanations on why Mabar hasn't consumed the domains of dread:

• It takes hundreds of thousands of years for Mabar to bring a piece of land from the hinterlands, and consume it. Sure, from the perspective of a mortal, that's a lot, but... Is that really the case for a plane that's been around since the dawn of creation?

• Time dilatation: Perhaps, Time is faster in specific places of Mabar, (because planes), so Maybe from Sthrads pespective, he's been around for thousands of years, but in Eberron, he went missing a couple hundred years ago.

• The domains of dread are the manisfestation of the memory and imagination of dark power. So, while Sthrad has been around for a long time, Barovia's is only a memory, a shadow of itself. Which would explain why most people don't have a soul in the domains of Dread.

• Every layer in Mabar is a domain of dread. The city of Amaranthine, the kingdom of Bones, and the last desert, are all domains of dread. Which would mean all layers were once a part of something, but Mabar chose to crystalize it, becoming part of it's metaphysical machinery of entropy, instead of consuming it. Maybe the city of Amaranthine WAS Irian's city of Amaranthine, stolen from Irian. The present one in Irian was rebuilt to look and be the same.