What made the Play Station 4/XBOX to have hard time running ACUnity?

Assassin's Creed Unity was released few months ago but it was really embarrassing for the PS4|XBOX gamers. The above mentioned consoles didn't even clocked a proper 30 FPS before there were some 40GB patches. What thing had gone wrong or is it the power of ACUnity that it made the consoles to sweat hard?
