I think there are many reasons that explain why bitcoin is better than cash:
+ At first, We don't have to waste paper to print cash.
+ Second, Cash could be faked, Bitcoin is not.
+ The last, Governments can print cash if they want, they couldn't do with Bitcoin.
The second and third points are very important. But you need to remember that although Bitcoin can't be counterfeited, it can be cloned. The examples are the recently listed Bitcoin Cash and soon-to-be-listed Bitcoin Gold. In the next 2-3 years, we can expect dozens of such Bitcoin clones. And I am not sure about the third point. A hard fork can modify the 21 million limit, right?