Its becase eth network has a huge traffic right now.
In case of bitcoin you don't care if you pay fee higher than other users. In case of eth as far as i know if you are single entity like shapesift all you transacions have order and and executer in order. What it means: if yours 1st transacion fee is to low to be process by the network, then all other transacions of yours will have to wait, even if they have paid enough fee.