If carbon has a versatile nature of combing with atoms of different elements then why it is present in such meagre amounts in the earths crust and atmosphere?
Are you sure the amount of carbon is meagre in earth's crust? Are you including the biosphere which includes all the bacteria, virus etc in the crust? If yes then the amount of Carbon will seem not so meagre. Remember we take
all our lifespan maximum to the extent of g to kg of other elements except Carbon, Hydrogen, Nitrogen, Oxygen, Phosphorus and Sulphur. These we take as much as we can. 100g per meal of Carbohydrates meaning roughly 40 g of Carbon per meal.
If you are talking about the crust of earth without biosphere then all the fossil fuels which we are using line graphite, Coal, Coke, Petroleum recently Shale oil all of them are Carbon compounds only. The total quantity of these will surpass the other elements any day.
The amount of unreacted carbon is less in the earth's crust mainly because of its reactivity and the ability to be consumed by plants and other organisms.