The main reason for the unconfirmed transaction connected to insufficient fees. All bitcoin transactions require a miner fee to be confirmed by the bitcoin network. When sending bitcoin you will be prompted to add a miner fee to make sure the transaction gets confirmed. If an insufficient fee is included, your transaction may never confirm.
The network is experiencing high volume
Sometimes there is a high volume of digital currency being sent globally, and there are more transactions than there is space available in each new block to include the transaction.