Hi,
VOLTE is cheaper than normal CS fall-back option. CS fall back require CS BW and LTE BW, but if you see VOLTE they do not need much BW and no more intelligence of fall back mode. you will get charged only once for all services you avail, maintaining different service charge is another headache for operators. If an operator dont have CS coverage then you cannot fall back for the voice call, still in some rural places is not having good coverage so if you want to deploy LTE with CS fall-back then you have to deploy 2G/3G as well, so you can reduce investment by making VOLTE.