Maybe calcium carbonate lends itself to big scale mounds because it is easier to pile up than other stone nearby? Both easier to split and easier to carry than say, the silicone solidity of sarsen? For some reason I'd always assumed the pyramids were sandstone, with only the outside having been limestone.

http://www.touregypt.net/featurestories/giza.htm
reckons the core was limestone too. Also that "kher neter" means "the necropolis".