A lot of people first get a credit card when they start their jobs or career and at this time their income is low.
This leads to the initial limit being sanctioned by the bank to be a small figure.
The bank also might not have any experience in dealing with a customer which could lead it to keep the limit lower to reduce its risk.
The credit score for a new customer might either not exist or it could be low which leads to a lower limit.
The initial amount is just the starting point and this can rise over a period of time.