There are usually three main reasons for this:

1. Previous record of bad credit.

Many banks won't issue a credit card to an individual that has had bad credit in the past. It doesn't mean that the person will never be able to get a credit card though. Getting store credit or small loans, and paying them diligently and on time, will increase your credit score. After some time of keeping a positive score, the bank might consider giving you a credit card.

2. No previous credit record.

If an individual has no credit history, the bank usually would not issue them with a credit card, as they have no knowledge of how diligently the user might be with payments. The risk to them is lower if someone has a history of paying off their debts on time, and in full.

3. Insufficient monthly income or no income.

The bank has to be certain that they will be able to receive payment from an individual. If you do not have an income, they won't issue you a credit card, as you have no means of paying it back. In many cases they would deny people with large sums of money saved in the bank too, as that money could be transferred or used at any point in time.

Similarly, the look at your monthly expenses compared to your income. If they find that it would be unlikely for you to make payments because of other debt and cost of living, they also won't issue you a credit card.