Why is South Africa written as RSA in cricket matches?

for the better understanding and it is an international game 

It is written as RSA because R stands for Republic, S stands for South, A stands for Africa.

RSA stands for "Republic of South Africa". The South African Cricket Association have this name as their abreviation because it is a status they have achieved in recent history. Just like how America, uses its tag 'USA' or as The United Kingdom of Britain and Wales is known as 'UK' and Russia was once called the 'Soviet Union'. They all refer a status or the form of Governance they achieved.

RSA is an abbreviation for Republic of South Africa.