Answer from cs61c-fz (Sean Nyberg 16014383) for Question 3
Because when we pipeline the data-path, we create a 5-stage pipeline. The whole concept of pipelining relies on being able to utilize all components of the datapath simultaneously for different instructions so that no component is not in use when it could be. By merging the data memory and the instruction memory into one module, that module can only be used by one instruction at a time instead of the data memory being used by one instruction and the instruction memory being used by another one. Thus the pipeline is destroyed.