Because the Boswellia tree is only found in certain African forests, notably the Yemen, Oman and Ethiopia. This gave it a scarcity value equivalent to gold.
Frankincense remains comparatively scarce today. Recent reports suggest it 'faces an uncertain future'.
Myrrh is another resin from a different tree (the Commiphoria). It is yellow, waxy and glossy. Like frankincense, it is largely found found in the north of Africa.
Myrrh is used today in many products including toothpaste, medicine (particularly Chinese medicine) and perfume.