I'm just guessing but I imagine the recent crypto bull-market has resulted in a huge increase of bitcoin transactions.
Bitcoin can only handle 7 transactions per second...
With a withdrawal from an exchange, it is likely that they paid the minimum fee for the tx to be included in the bitcoin blockchain.
If you could manually tell them to pay a higher fee, your tx could have jumped to the head of the queue.
TL;DR: BTS > BTC