It is standard issue, your transaction fee was not high enough. Current fee needed for fast confirmation is 270 satoshi/byte your transaction had 120 sat/byte.
Another thing is problem with bitcoin network, it is overloaded with pending transaction. ATM we have over 170k unconfirmed transactions.
https://blockchain.info/unconfirmed-transactionsYou can use ViaBTC accelerator or this tool
https://www.btcspeedy.com/#accelerator to speed up your tx.