There are two primary areas that are may be the issue. One is the method used to make all Apple Pay transactions safe, biometrics (Face ID, Fingerprint ID). Some governments don’t permit biometrics to be used for financial transactions. Apple’s claim is it’s more secure and a fundamental part if making Apple Pay safer. Some countries banking laws require the PIN. Apple Pay does not utilize a PIN for credit card transactions.
The second issue is the storage of data. Some governments don’t want data stored outside the country. Apple servers are in the US. Even though all the data is encrypted and Apple does not have the key to decrypt, this runs contrary to some countries financial laws.
Lastly, there is the issue of the infrastructure the banks in the country need to implement. if the banks don’t see the financial advantage of adopting Apple Pay, they have no motivation to install the hardware and software required to implement Apple Pay.