Mongolians use the birch wood for core construction and siyah as well.
Birch is a good wood, so is maple wood too. Mountain maple was considered excellent by other makers, superior gluing properties and quite elastic.
I can see why mulberry is big choice too.
Really, few country use bamboo for core, they dont like it. I think only Korean bowyer regard it high.
It is light weight, but splits.Used more for mass produce. Still, you can make good bow with it.
Light, elastic wood with good shear strength and gluing abilities is what you need for core. It does not need to be thick, so its not very big factor.