Grit Deck Help

So as the topic says, I am unsure as to why grit decks such as Asajj Ventress I work Alone fail so often. Statistics tend to be on your side by pining your units making them stronger and weakening their own. Can someone explain to me how and why these types of decks don’t usually do well.

*Side Note - I am seeing success with Qi’ra but am still left confused.