Because it's more complex under the hood. We send money to a target account and if it's in another bank, the bank forwards that to a centralized clearing system that can take a day or 2 to settle. It requires an authority to slap the hands of those who would default on their settlement and there are regulations to go through to access this system.
For example in Canada : https://www.bankofcanada.ca/core-functions/financial-system/canadas-major-payments-systems/
That's for canadian settlements. It says in there that if the sender defaults, the receiver absorbs the loss. Now immagine international settlements.
With a developed futures market in each major currency, you can do international settlements in Bitcoin directly without a central authority and without the risk of transaction rollback. The receiving party will use their currency of choice futures to lock in the rate. At any point they can sell their bitcoin at spot and close their futures to receive their local currency.
