You can find a good explanation of why it was replaced by reading A name for the null pointer: nullptr, to quote the paper:
This problem falls into the following categories:
Improve support for library building, by providing a way for users to write less ambiguous code, so that over time library writers will not need to worry about overloading on integral and pointer types.
Improve support for generic programming, by making it easier to express both integer 0 and nullptr unambiguously.
Make C++ easier to teach and learn.