Why is my Bitcoin transaction pending for 24 hours?

I'm curious to understand why my Bitcoin transaction has been pending for an entire 24 hours. I've initiated the transaction with sufficient funds in my wallet, and the recipient's address is correct. Could it be due to network congestion? Or is there a specific reason why my transaction is taking so long to be confirmed? I've heard about transaction fees, but I've set a reasonable fee based on the current market conditions. Is there anything else I should be aware of that could be causing this delay? Any clarification or insight into the potential causes would be greatly appreciated.