Nothing, as far as I know.
Take a look at the JAX-RPC article on Wikipedia, first sentences says:
Java API for XML-based RPC (JAX-RPC) allows a Java application to invoke a Java-based Web Service with a known description while still being consistent with its WSDL description. It can be seen as Java RMIs over Web services. JAX-RPC 2.0 was renamed JAX-WS 2.0 (Java API for XML Web Services).
I suspect that the "RMI over web services" idea was used when devising JAX-RPC and RemoteException was introduced since RMI use it.
Also take a look at the RemoteException API documentation - it is the parent class of a number of communication-related exception classes, so it has served, and in some areas still do server, as a general way of saying "there is a communication problem" with details specified by subclasses.
As usual, I open my mouth without better knowledge - since I haven't been involved in developing JAX-RPC, I can just speculate.