The number of card-accepting merchant outlets worldwide rose more than 7 million in 2015 to 54 million, according to "Global Payment Cards Data and Forecasts to 2021," the latest research from RBR.

The report notes that growth was especially strong in underserved markets including Asia-Pacific, the Middle East and Africa, and central and eastern Europe.

In Asia-Pacific, the number of merchant outlets now accepting payment cards increased 29 percent in 2015, to 25 million.

China accounts for much of this increase, with 4.7 million new outlets adding payment card acceptance during the year. Underserved rural areas are seeing robust growth, RBR said.

Regulation is playing a role in card acceptance in several countries. In Malaysia, interchange fee reform has encouraged merchants to begin accepting cards. Legislation introduced in Kazakhstan would require all merchants to accept card payments or face a financial penalty.

Regulation is also bringing about further expansion in mature markets. Caps on interchange fees, which were implemented last December in EU nations, have bolstered the business case for merchants to accept card payments.

Technological advances are stimulating greater card acceptance, as well — even in developed markets. Contactless appeals to merchants that handle low-value payments and appreciate its shortened transaction times.

Additionally, mPOS devices are encouraging card acceptance among mobile merchants such as taxi operators and tradespeople, according to the report.