The main reasons why a credit card application may be denied include low credit scores, having too many recent credit inquiries, high outstanding debt, insufficient income to cover the payments, being underage or unable to prove independent income if under 21, having derogatory marks on your credit report like late payments or bankruptcies, and applying with a frozen credit report. Other factors include not being on the electoral roll for identity verification, limited or no credit history making it hard for lenders to assess risk, unstable employment history, mistakes on the credit report, or financial associations with someone who has poor credit history. Ensuring accurate application details, maintaining a good credit score through timely payments, reducing existing debt, and verifying income can improve chances of approval. If rejected, credit card issuers are required to send an adverse action notice explaining the reason for denial within 30 days, which helps to identify specific issues to address before reapplying.