The reason, he says, is volatility. The value of Bitcoin and most other digital currencies fluctuates by large amounts. Just this year, one unit of the cryptocurrency has swung in value from less than $5,000 in March to nearly $14,000 this week.

“Bitcoin per se is volatile in its valuation,” Banga said at the Fortune Global Forum conference on Tuesday. “Can you imagine someone who is financially excluded trading in a way to get included through a currency that could cost the equivalent of two Coca-Cola bottles today and 21 tomorrow? That’s not a way to get them [included]. That’s a way to make them scared of the financial system.”

People without bank accounts lack access to credit and pay much higher fees for financial transactions through payday lenders and the like. Five years ago, the Mastercard CEO committed to help 500 million people gain access to the financial system, and this year he upped the goal to 1 billion people. That has led to programs like partnering with Save the Children Yemen and World Vision on voucher-based payments programs in Africa and Asia.

Many cryptocurrency supporters argue that the lower cost of using digital currencies to buy and sell could also be useful in getting more people into the financial system. In an essay earlier this year, CoinDCX CEO Sumit Gupta proposed using digital currencies as a “panacea for the unbanked,” giving people access to funds and transactions without a banking infrastructure.

Still, Mastercard hasn’t ignored digital currencies altogether. Banga says the development of more stable cryptocurrencies by central banks could be an improvement. Such currencies, which are under discussion, would be backed by each country’s existing currencies, known as fiat currencies.

“Fiat currencies, if they were to go digital, would they be helpful in cross-border trade flows and improving the efficiency of those—yes for sure,” Banga says.

Last month, Mastercard created a testing environment for helping central banks develop digital currencies.