Please don your tinfoil hat - all that follows is speculation based on in-game evidence. It is not canonical lore endorsed by Blizzard.
One of the big reveals of Wrath of the Lich King is the Curse of Flesh. Upon our arrival in Ulduar's Halls of Stone, we escort Brann Bronzebeard to the Tribunal of Ages, a repository of Titan knowledge. After a fierce battle with the Tribunal's defense systems, Brann manages to access the Tribunal's information and learns the history of Azeroth, including how the Titans created Azeroth and how the Old Gods came to infest it, and how the Titan's creations of stone and iron were infected by the Curse of Flesh, making them more easily assimilated by the Old Gods. After defeating and imprisoning the Old Gods, the Titans re-engineered their creations to ensure they were no longer susceptible to the Curse... leaving the ones they'd already created to suffer it, and slowly change into the dwarves, gnomes, humans, troggs and their offshoots. Thus was Azeroth peopled in many cases.
It sounds plausible enough. But there are some problems with it - namely, not all of the Titans information sources agree with it. For instance, the first Titan trove accessed by the mortal races of Azeroth was in Uldaman, in the Badlands. This Titan complex, lying in the heart of the Eastern Kingdoms, is potentially the source of the dwarves and gnomes who live nearby in the mountains of Khaz Modan.
The Lore Keeper of Norgannon we meet at the end of Uldaman tells us that the Titans deviated from their normal plan when creating seed races.
What does this mean? Rather than the typical biomass construction foundation used by the Creators implies that the use of stone and other materials in the Titan constructs of Azeroth is not standard. This is not what the Titans usually do. Why did they do it on Azeroth, then? They appear to have done it quite extensively as well - the Earthen, the Mechagnomes, the Vrykul, the Mogu, the Tol'vir - a whole host of inorganic entities, using 'a cross-section of Azeroth's crust' to construct them. And why is the resilience of Azeroth's essence so remarkable?
A cross-section of Azeroth's crust was used as the foundation for the Earthen's synthesis rather than the typical biomass construction foundation used by the Creators.
Research on the world's composition led the Creators to theorize that an enhanced being could be synthesized that would epitomize the resiliency of this world's essence. This was accomplished by choosing to use a blend of Azeroth's various stone core compounds as the foundation.